Short answer: A lot.
Most common reasons noted from a guy who has been rping for ten years?
#1: Interest spikes and dies. common for the primarily high school based population of Roleplayer Guild.
#2: One or more players quit out of spite against a GM whose decisions they didn't like or because of another player, causing a chain reaction.
#3: Nobody has any idea what they are doing story-wise and everyone suffers for it.
#4: Because nobody takes action in the story with their characters: They comment on what has happened, giving less total actions to comment on next time, until this compounds into an implosion when the RP can no longer sustain their dead weight.
#5: Especially in Advanced, people scared of posting subpar material never feeling satisfied with their posts. Thus they never post. Back to compound implosion.
#6: Because the story ends up filled with characters written like main protagonists instead of party members. (aka: Everyone is Commander Shepard syndrome.)
#7: Because the GM and/or players never stop thinking about new ideas, thus causing old ones to die from split attention. (ADHD syndrome.)
#8: Because bad luck. Happens to everyone.
#9: Because most people are too arrogant to admit that their writing skills may be flawed. Ergo they usually write themselves into corners, make scapegoats and excuses, and quit instead of learning how to better write next time.
#10: Because people are abhorrent at managing time and get caught up in other activities, such as education, or gaming.
I'm guilty of some of these on occasion too, so no, I'm not writing these up as some elitist who claims to be exempt from these.
Generally speaking, the only solutions are to...
#1: Through trial and error, find a core group of players/friends you can depend on to consistently post, who share a similar taste in role plays such as you.
#2: Constantly self-evaluate your own work. Make sure to always leave ways for other players to interact with your character(s), and make sure you're always seeking out interactions with others.
#3: Maintain interest. Be focused. When you aren't writing a post, think about potential ways the story could go from there. That not only helps to keep your mindset on the story, but gives you potential routes you can pursue when you are ready to write your next post.
#4: Never join more RP's than you can handle, and always have enough time to keep up with whatever pace the group has set. This is usually where I fail most often.
#5: Never be afraid to post even stuff that you consider subpar. It's better to have a mediocre post to work with than no post at all.