The silliness/seriousness balance varies by audience, but the working balance is typically in the middle, wedged slightly to either side. Horribly serious games can potentially get boring, but also attract a very small audience even though they have the potential for fantastic high drama. Absurdly silly games have a wide level of attraction but very little pull once you arrive. They're fun, but you can pick them up as easily as you can put them down, contrasting very serious games which have little attraction but a heavy pull once you're in.
Next up is buy-in. Suggestion adventures have a greater staying power than straight stories because players get a sense of buy-in. They also tend to be fairly open to the public, rather than closed games with high buy-in like Godhood. Higher buy-in leads to more survivability.
Content is a powerful draw, especially high level content. Elves of Amanereli has the draw of both good writing and a beautiful art style. DarkerDark's art brings people back and keeps people waiting even after months-long absences. The fact that he is a sole creator helps in that you don't get multiple person burn-out. You are King's content (with its high buy-in as well) comes from the high base quality of writing and storytelling provided, as well as an insanely fleshed out (and perhaps definitely overly complex) ruleset to back it up. The ruleset content of You are King stops it from degrading into "Cops and Robbers" freeform adventuring where nobody can agree who got shot and who did the shooting.
Burn-out and regularity are very important. Space Adventure! has a big draw because even though the art style is good but not fantastic (and I think Robocorn is humble enough to agree, when compared to DarkerDark, but that doesn't mean it's not good in its own right) Robocorn updates regularly - not like clockwork but often enough that people know they can come back and not find it going on a month-long hiatus.
Burn-out is an issue with game-mastered games. Where managing the responses to ten or twelve players is put upon one person's shoulders (especially if those responses are on the high content level of Godhood or You are King) a skilled and enthusiastic GM can keep a game up for several months but will eventually burn out and get sick of it. Sooner if they don't have an insane level of dedication (which most people are smart enough not to). The prime success of the first part of You Are King was inheriting the (accidental) twain-GM format of You are Lord, but a certain creator was foolish enough to put far too much energy into it and burned out nonetheless. The sequel preserves and continues in part because there are at least 3 GMs now and because they are all very balanced, sensible individuals. (Hats off to CG in particular.)
GM Interest can also be an issue from the start. When a game begins with a clear indication of either high player involvement (choose your own adventure!) or high quality content (3 paragraph intro with excellent artwork) and a well-established humour:drama ratio it will have a powerful initial pull as it leads to expectations that it will either be easy to continue or the creator is very dedicated. When the game seems devoid of content or at least creativity for the players it seems to display a lack of interest on the part of the creator - such a game is doomed to failure.
Some games will find their way back no matter what, though. You are WIZARD! being a particular contender...