To be overly simple..

The NFL is VEEERY quick to flag for any hit anywhere NEAR the head, hence the lower extremity shots......On top of what I mentioned above. YEARS ago, while in the USAF, I TRIED to bulk up. I had a trainer tell me the workouts alone to bulk up are BRUTAL BRUTAL to knees. Contact is a whole other issue......And that's when O line guys were HOPING to reach 300 LBs......That's "small"these days. In the quest, something has to give....usually ACLs.