there is an unspoken rule of not emulating games newer than five years. in some cases, alot of games use the same basic system, like bemani games, and thus, none of those games will be emulated until the last one made is five years old (at least).
also, it depends on if the mame team has a game to dump or if someone is up to doing it. some games don't get done right away because it isn't available or because no one has gotten around to doing it.
there is alot more to all this, but this is the simple reason.