Some reasons for not including it I can come up with:

• It doesn't run on all platforms and/or architectures supported by the perl core, so tests will fail
• It doesn't offer anything that cpan does not already do (enlighten me if I'm wrong. I don't mean cpanm does it faster or easier or nicer. I mean something that isn't possible with cpan and is a killer feature everyone should be using)
• Nothing in the core depends on it, so it doesn't add to features of perl itself, only to (some) user experience
• It is extremely easy to install after perl itself is installed (on supported systems)
• It does not (yet) fully support distroprefs, something vital for distributors
• It add maintenance costs and testing time
• It might be replaced with something even better