Oculus founder says no Mac is powerful enough to run the Rift
techspot.com
techspot.com
http://www.macobserver.com/tmo/article/the-new-mac-pro-is-a-...
It was a decision by Tim Cook, and it made it so the Mac couldn't compete in gaming next to a PC, Tim Cook most likely made the decision to save money on the Mac Pro design.
In fact older Mac Pros had limits on what video cards were compatible: https://support.apple.com/en-us/HT201805
If Apple brings back the Mac Pro ATX Tower, it could have a gaming Mac again. Until then people are going to build a Hackintosh to game on OSX.