Because DX11 is about to lose all support, so instead of pushing this onto people RIGHT AT THE START OF BFA they decided to do it now to tune it correctly and possibly fix bugs that are required.
DX12 is what they need to change to, not their fault that its losing all support. Everyone here would be quick enough to complain if it became riddled with bugs and when people say "omg why can't blizzard fix this omg" for them to inform you it's not supported anymore.