If you have 100 different libs that are basically "standard" (who doesn't have `mtl` in their applications at this point), now you have to coordinate 100 different library updates roughly at the same time. If you forget even one of them, then you've broken everything.
I think the argument for a large Prelude/standard lib is similar to Google's "single repo" argument: Easy to catch usages and fix them all at once. Plus you're making the language more useful out of the box. People coming from python can understand this feeling of opening a python shell and being productive super quickly form the get-go.
Arguments for small std lib exist, of course. But Giant standard libraries are more useful than not.
EDIT: I think the failure of the Haskell Platform has a lot more to do with how Haskell deals with dependencies, and the difficulties it entails, than with the "batteries included" approach itself.