Umm, 2004 olympics, leg grabs were legal. I don't recall BJJ stance being dominant. Also, as mentioned before, lowering your base and crouching over are not the same thing rounded back crouch without lowered base just means shitty posture, and does fuck all to defend you from double legs
It's the scoring more than the restrictions, although they have since added those too.
You're probably not getting ippon off a shot. But in BJJ all takedowns are the same, so you see more shots to the legs.
Crouching is just better overall when takedowns aren't scored according to amplitude. That's why it is dominant in BJJ and Wrestling.