In high school and college, though I can't speak for the older demographic, listening to this "techno" (outside of playing Dance Dance Revolution) was scorned, and labeled you as some cross between a nerd gamer, weeaboo (e.g. anime lovers), and goth (not makeup-wise, but reputation- and fashion-wise). It really was a "closet" genre as OP put it. I kept my Kazaa / Morpheus (pre-BitTorrent) finds to myself.

In high school and college, though I can't speak for the older demographic, listening to this "techno" (outside of playing Dance Dance Revolution) was shunned, and made you some cross between a nerd gamer, weeaboo*, and goth (not makeup-wise, but reputation and fashion-wise). It really was a "closet" genre as OP put it. I kept my Kazaa / Morpheus (pre-BitTorrent) finds to myself.

^{* This may not be PC, but kids were pretty brutal in the 90s. I've been called much worse for listening to electronic music.}

tl;dr - Miley Cyrus, FL Studio, Sirius XM, and electronic music festivals.

(First one's to get your attention, but not entirely untrue.)

2020 Update: Something I didn't realize when I wrote this answer in 2016 is that this experience—of electronic music going from shunned to mainstream—is specific to a narrow generation of those born in the late 80s and 90s. By the 90s when we old enough to explore our identities in music, radio and MTV had already shifted to grunge, rock, and hip hop. The entire era of synths and electronica preceding the 90s was invisible to us, except maybe through our parents (but mine were immigrants, for example). So indeed to us it was a surprising shift, seemingly from "non-existent" and abused for listening to it, to a social mainstream.

So in this post, I'm talking about EDM. Not Prodigy, not Aphex Twin. Not The Postal Service, or Royksopp. All of which one might consider "electronica". I'm talking about "four-to-the-floor," danceable electronic music, whether one keeps their hands straight down and stomps their feet with a cigarette in their mouth or holds their hands up through 128 measures of euphoric breakdowns.

Maybe that wasn't the case everywhere, though. I grew up in upstate New York, and the data points I have outside of this region is a friend from NYC and another from Des Moines, all of whom experienced the uncoolness of techno. But perhaps in a city like Miami, which has hosted Ultra Music Festival (UMF) since 1999, or cities with huge entertainment industries like Vegas, NYC, and LA, there might have been less of a stigma against techno. I don't know. Regardless, I still don't think nightlife was dominated by techno.

For an adolescent growing up in the 2000s, "going to the club" meant: hip hop and punk. Wasn't it about the same for adults, though? Sure, I know techno venues were available, like the legendary Limelight in NYC. But I don't think they were mainstream. In 2016 now, when my friends want to go to "a club," chances are there'll be some DJ from Berlin spinning minimal or tech house, wherever we end up going. I don't think this is how things were 10 years ago. [Coming back to this post several years later, I think I missed something. I think the mainstream shift to grunge, rock, and hip hop in America naturally caused kids born in the late 80s / 90s (I'm '87 and I'm guessing OP is '88) to never have experienced the electronic era that just preceded them. That's why it felt so stigmatized amongst our peers and why it appeared completely new in the 2010s, to us.]

That's why I don't believe it's just post-college migrations or what every age cohort goes through. The mainstream changed dramatically in America, EDM coming to [re-establishing itself in] the forefront of nightlife as well as mainstream outlets, and becoming accessible to a demographic that includes grade school and college, finally busting the 2000s' stigma against techno.

In the early 2000s, amidst a lot of R&B, boy bands, this guy, there was Britney Spears, Christina Aguilera, Outkast, and such artists whose music revived a bit of the "dance" aspect in mainstream. Maybe not quite four-to-the-floor, but the production was increasingly picking up elements that already existed in "modern" electronica: compressed kicks, percussive delays and verbs, synths and pads. There was also Daft Punk, though, in my neck of the woods, they didn't get big until their Discovery album in 2005 (and gained another surge of followers post-Kanye); and Eric Prydz's Call on Me, which played about 5 times a night at any given fraternity. I believe that both of these artists, while trying to go "retro," helped bring the future.

I think it's possible that these developments in the early to mid-2000s may have "primed" Americans—even those who don't typically listen to the radio—to embrace a more energetic and constant style of music, in contrast to rocky, funky, or hip-hoppy variants also competing for the mainstream of that era.

In my last year in college, 2008, I was doing homework with a friend, when she put on Miley Cyrus's "See You Again". If you play it, you can hear what an intermediate this is between the budding semi-EDM mainstream of the 2000s and the EDM we have now—besides the four-to-the-floor, the "and of" kicks reminiscent of techno, the saw bass (nowadays mostly recognized in dubstep).

tl;dr

How did electronic music become popular in the early 2010s?

Miley Cyrus, FL Studio, and Sirius XM, and electronic music festivals.

First one's to get your attention, but still kinda true.

^{2020 Update: Something I didn't realize when I wrote this answer in 2016 is that this experience—of electronic music going from shunned to mainstream—is specific to a narrow generation of those born in the late 80s and 90s. By the 90s when we old enough to explore our identities in music, radio and MTV had already shifted to grunge, rock, and hip hop. The entire era of synths and electronica preceding the 90s was invisible to us, except maybe through our parents (but mine were immigrants, for example). So indeed to us it was a surprising shift, seemingly from "non-existent" and abused for listening to it, to a social mainstream.}

So in this post, I'm talking about EDM. Not Prodigy, not Aphex Twin. Not The Postal Service, or Royksopp. All of which one might consider "electronica". I'm talking about "four-to-the-floor," anyone-can-dance-to electronic music.

I was in upstate New York though, so maybe that wasn't the case everywhere. So I interviewed a friend from NYC and another from Des Moines. Both experienced the uncoolness of techno. But maybe in a city like Miami, which has hosted Ultra Music Festival (UMF) since 1999, or cities with huge entertainment industries like Vegas and LA, there might've been less stigma about techno. Regardless, I tend to believe that nightlife still wasn't dominated by techno.

For a teen growing up in the 2000s, "going to the club" meant: hip hop, maybe punk. In 2016 now, it means a DJ from Berlin spinning minimal or house, anywhere we go. [2020 update: And now it's techno.] That's why I don't think it's just post-college migrations, something every age cohort goes through in the same way.

Mainstream changed dramatically in America, EDM coming to the forefront of nightlife as well as mainstream outlets, and becoming accessible to a demographic that included teens, finally eradicating the 2000s stigma against techno.

In the early 2000s, amidst a lot of R&B, boy bands, this guy, there was Britney Spears, Christina Aguilera, Outkast, and others whose music revived a bit of the "dance" aspect in mainstream. Maybe not quite four-to-the-floor, but the production was increasingly picking up elements that already existed in "modern" electronica: compressed kicks, percussive delays and verbs, synths and pads. There was also Daft Punk, though, in my neck of the woods, they didn't get big until their Discovery album in 2005 (and gained another surge of followers post-Kanye); and Eric Prydz's Call on Me.

I think it's possible that these developments in the early to mid-2000s may have "primed" Americans—even those who don't typically listen to the radio—to embrace a more energetic and "constant" style of music, in contrast to rocky, funky, or hip-hoppy variants also competing for mainstream in that era.

In 2008 I happened to hear Miley Cyrus's "See You Again". In it you can hear the budding semi-EDM mainstream of the 2000s as well as the EDM of today—besides the four-to-the-floor, the "and of" kicks reminiscent of techno, also the saw bass (nowadays mostly recognized in dubstep).

TL;DR

Miley Cyrus, FL Studio, Sirius XM, and electronic music festivals.

First one is just to get your attention :^)

tl;dr - Miley Cyrus, FL Studio, Sirius XM, and electronic music festivals.

(First one's to get your attention, but not entirely untrue.)