[jrockway]

Finally, all of hackage is a 'cabal install' away, reducing the need for a large core set.

This I think is the most important point. The value that the Haskell Platform has is that it includes the stuff that depends on system libraries (X11) and other packages that are more difficult than average to install. That means that once you get the Platform, everything else will probably be easy to install, and so there's no point in including it.

(This is especially true of things like web frameworks; what's popular today may not be in 5 years, and deprecation cycles are hard. Perl still includes CGI.pm, and people still use it because "it's core". Compatibility is nice, but encouraging obsolete programming techniques is not.)