I think on E my biggest game has been around 15 people, including me as I always play a character too. Most of the E group games I see dive to around 2/3rds or 1/2 their starting number of players within a month though. Most die within 2 months. Only 2 of my many group games have lasted any length of time, though I am in one now that has been going a year, run by someone else, though its pace is terribly laid back, with gaps of up to a month between posts sometimes.
Talking generally about why games on E die I think its mostly due to:
1) Fickle membership, of a mostly young mobile demographic (still at school - don't ever tell me everyone here is over 18) and subject to various things that pull them away from the site such as studying and general growing up issues, relationships, internet connection woes and costs, etc. Note the 'fickle membership' problem applies equally to GMs too. I've been in several games that have been abandoned by its GM resulting in a quick collapse. I have found that the most reliable players are the older ones, aged at least around 30 or more who have stable jobs, relationships and financial situations.
2) Too great a choice of games. There is little to keep a player in a game when another apparently great one starts up the following week... and so on.
3) Friction between players or between players and GM. Sadly I know I cause a few people to leave my games, or I eject them early on as patently unsuitable. I'm a pretty harsh and demanding GM but equally I find too many players on E are too thin skinned and immature to take criticism, and won't (for example) edit an unsuitable character.
4) As Chrystal said, game concepts that appear brilliant on paper but clearly are not so brilliant as group forum RPs and die fast due to various unforseen technical or plot problems.