I don't see why WoTC couldn't reboot the meaning of "core" during all the playtest and previews. So then the current players are "re-educated" and the newbies get a word that inuitively works for marketing purposes.
Because it holds too much baggage and people are resistant to change. That, and some people like to remain willfully ignorant.

The word "core" is too far gone to be of any use anymore. Retire it. Like Crazy Jerome did... just go to thesaurus.com and choose ANYTHING else to replace it. It'll be much easier and much more palatable than trying to rehabilitate "Core"'s image.