While I can agree that there's plenty of flaky GMs out there (this has far less to do with experience and more to do with them lacking depth and dedication, neither of which will [i]truly[/i] be taught to you by experience), I find that another reason why the role-plays die a lot of the time is simply because the players lose interest or become 'busy'. Since I'm confident in my own abilities as a GM, it's become my first priority when creating a role-play to gauge the dedication and integrity of those wishing to be my players. I figure that those who're unwilling to go through the character sheet process (for example) probably aren't going to have the dedication for the role-play. I think the single most problematic thing that kills a role-play is people being whimsical. This is also why I generally create role-plays that are capable of surviving if one of two players bail on us.

Also, some people don't find it as easy to just view the role-play as a journey. Not only is it quite difficult to be satisfied with a journey that barely even started (which appears to be the mainstream of this site), but it's also a lot harder to be satisfied with a role-play coming to an end if you're the type of person (like myself) who doesn't find role-plays they're interested in easily. People who're genuinely dedicated to role-plays usually pick a thread because something about it clicks with them, and often go out of their way to create a character that means something to them too (which may or may not fit well into other role-plays). Simply watching all that fall apart and then needing to just look elsewhere can be very disheartening, and - frankly - I question the overall devotion as a player/GM of someone who could simply move on that easily.