1. the acceleration due to gravity g = G M / (Re+h)²
Gravity is less at an altitude h from the surface of Earth. Hence, the ball bounces higher. Suppose the ball rebounds with a velocity v.
S = distance it rises to = v² / 2 g.
As, g is less on mountains, S is more.
2. Also, air pressure is less. The drag resistance due to air is less. So the ball bounces higher.