Board Thread:Warhammer 40k General Discussion/@comment-94.171.175.196-20131022124517/@comment-5.67.216.112-20131022205935

I think its because they've put them all into one single codex, they needed a way to show that they are Space Marines... but different.

As for the Librarians, I think its a more sensible explanation than just 'They dont like them'.