I usually only take part in 1x1's simply because I hate having to wait for five other people to post. 

However, with that being said, in the past RPs I have done that were group games, they were in the advanced section. If the GM disappears and there is no Co-GM, then it dies unless everyone is in agreement to continue it on their own. Even then, there has to be a captain, and the person who takes over may not take it in the correct direction. Speed posting, leaving other characters/roleplayers out, fights breaking out between writers, different people having different styles of writing that don't work well together, and just a general loss of interest by all parties.