As I said Thanee, I might be wrong. I'm only doing pbp games for a month or so.
One of my games has a DM who writes very elaborate (and nicely written I might add) posts as well as players who make an effort to pitch in additional details, dialogs, monologs, description, whenever possible. And you know what. Its infective. I find my self giving it all I can as well in that game, I'm motivated to post the same quality stuff the others do. Not only that, I find it easy to do. There's always some quote, some nitpick, some new setting detail that I can include in my posts. Actually, there's more than I can usually fit in one post.
The other game has a DM who's going for a very laid back style of gaming, basically spelling out the bare essentials but never giving much beyond that. The game is certainly not slow, there are daily updates, but there is never any meat to the bones, so to say.
And consequently, the players mainly (with one exception) just go for one-liners as well, and I can not blame them. I myself find it hard to post anything useful in that game since there are no hooks or details to play off, no flavor to toy around with to fill a post with should there be no combat about. I already know for sure that this game will likely never see it's end, or if it does, that it'll be a drag all the way.
(including dreary combat sequences with page after page filled with more numbers than flavor text - and again, I cannot blame people for it, if there's no flavor, metagaming and numbercrunching's all there's left to do)
It may be the individual style of players, but it's certainly always afflicting the whole game in one way or another.