It's pretty common to see a band break up or have a member or two leave, but in Guns N' Roses the only original member left is Axl Rose and most of the other band members still play together in bands like Velvet Revolver. Is there a reason behind the departure of most of the original band members?
The simplest reason is that Axl doesn't have the best character (to put it mildly). Most of the members started disliking, even hating him after some years.
He was known for being at least 2 hours late for the shows, which as a musician and a fan, I find really irritating.
With 'original members' I assume you are referring to the Appetite for Destruction line up, which consisted of Axl (vocals), Slash (lead guitar), Duff (bass), Izzy (rhythm guitar) and Steven (drums).
The first to go (1990) was the drummer, Steven Adler; he was shooting dope so bad, he couldn't perform. While the band was recording 'Civil War', he needed 30 takes to get the song right. Thus he was fired.
The reason that Izzy (rhythm guitarist) left (1991) was because of the Riverport Riot in St. Louis, where during the show, Axl went down to the crowd and started punching a guy that was taking photos.
Also, as it is known, all of the original members were junkies; Izzy was the first one to quit the dope and he found it really hard to stay in the band when everyone else was shooting while he was trying to fix his life.
It was mentioned on Slash's autobiography that his relationship with Axl grew from bad to worse, due to the latter's misbehavior. During Slash's final years in the band, he was drinking and doing coke so bad, he was on the verge of death. That is the reason he quit. A few months after he quit (1996), he got his life back together and got off drugs and alcohol.
The last original member of the Roses to leave was the bassist, Duff (1997). His reasons were similar to Slash's, but also due to the fact that the band was largely inactive.