Exactly 17 people in history have reached 8 feet, 3 of them are alive today and none of them have the build to match the Mountain. I'd rather have a "shorter" actor who looks as overwhelmingly strong as Gregor is supposed to be than one the 'correct' height who doesn't.

I seriously doubt anyone that tall could be as powerful as the Mountain anyway, the human body doesn't really work at 8 feet high.