Much of the velocity on a home run comes from the pitch (equal and opposite reaction) rather than the bat speed. So it’s not that easy to hit a home run on a 50 mph pitch.
Incoming speed makes significant difference to exit velocity.
If you were going for max distance, you would want 100 mph pitches.
In home run derby, they are optimizing for ability to get a good repeatable swing with enough power to go over the fence. If they were playing in a park with like 440 ft fence, they would need faster pitches.
The distance the ball flies has a lot more to do with bat speed than ball speed (something like 5mph for 1mph, if I recall correctly).