10.12 on 5,3

It looks like 10.12 won't support mid-2009 MBPs, is that correct? If so, why not? Is there a major/core change that makes it impossible to run or is it just minor and it can be overcome?

--- Post Merged, Jul 31, 2016 ---

nevermind, should've searched before posting; found this http://forums.macrumors.com/threads/macos-10-12-sierra-unsupported-macs-thread.1977128/
someone delete this thread