Question

Why can't passenger planes fly very high?

Answer 1

The higher in the atmosphere, the less oxygen, and the more cold temperatures.

The material/moisture on the plane would freeze and damage the engine.

Answer 2

Plane engines need oxygen to burn fuel, they have to get their oxygen from the outside air. If they fly too high, the oxygen concentration in the air will drop too low for the engine to maintain combustion and will cause it to stall.