I've been looking into the bossa nova style and from what I've researched, bossa nova seems to have taken Brazil by storm when it was introduced. But I'm not entirely sure what the dominant style in Brazil was before bossa nova. A couple sources have implied that samba was very popular beforehand, but is this really true? It just seems strange to me that a nation would so easily go from a heavily percussive, flamboyantly-vocalized style to a relaxing jazz.
Samba was indeed the popular music in Brazil prior to Bossa Nova, and before that came Choro. However, it's worth noting two things:
Firstly, samba in Brazil existed in multiple forms, including not just the carnivalesque (and internationally successful) samba-exaltação, but also the slower, tragic-romantic (but equally overblown) samba-canção.
Second, Bossa Nova was partly a minimalist reaction against the vulgarity of much of 1950s samba (just like punk was a reaction against overblown prog rock). However, like for punk, it's important not to overestimate Bossa Nova's outreach. While it was hugely successful abroad (like samba-exaltação had been before), Bossa Nova in Brazil was significantly middle class and white, both in origin and consumption. The poorer population continued playing and listening to samba and other regional music styles, such as música caipira.