To me it's because off 1 you have more momentum going up and when you jump your other leg goes up so that gives you another bit of momentum and height too. Jumping off 2 you're either stationary or you have to slow down a lot to push off with both legs.

I can never grab the ring off 2 but could pretty easily when off 1 (I have never been able to dunk).

Also the same with rebounds. When you're running in from outside for a rebound you just jump for the ball off 1 and you can beat stationary bigs for the rebound. But if you're boxing out its hard to get as much height.

Of course this is just personal experience it is probably different for everyone.