[Haskell-cafe] Stupid question #374: why is MaybeT not in the standard library?

I wrote my own implementation of MaybeT (which was a usefull exercise), but a quick google showed: http://www.haskell.org/haskellwiki/New_monads/MaybeT But I'm wondering why it's not in the standard library. The standards committee just hasn't gotten around to it yet? Or was there some discussion of this in the past on some (public) maillist, that my admittedly shallow googling failed to uncover, that someone could point me at? Thanks. Brian