The reason I know of why mapM wasn't just made to be an alias for traverse (assuming that's what you mean) was that it was thought that particular definitions of mapM could be more efficient than traverse. For instance:
mapM :: Monad m => (a -> m b) -> [a] -> m [b]
mapM f = go []
where
go ys [] = return (reverse ys)
go ys (x:xs) = f x >>= \y -> go (y:ys) xs
This doesn't use stack for m = IO, for instance.
However, it has since been pointed out (to me and Ed, at least), that this matters much less now. Stack overflows are now off by default, and if you measure the overall time and memory usage, traverse compares favorably to this custom mapM. So, as long as stack isn't an artificially scarce resource, there's no reason to keep them distinct. We didn't know this until after 7.10, though.
If you're just asking why the definition of 'mapM' for lists isn't 'traverse' with a more specific type, I don't know the answer to that.
-- Dan