Question
homework question...Why do trees never grow above a certain height??

Answer
I assume you mean height as in tall not altitude as on the top of a mountain. It is a combination of many things--soil type, the climate, tree genetics, species of trees and the age of the tree. Different soil will grow tree to different heights-we Foresters call this site index--the height a tree will grow in 50 years on a soil.
The climate has a play in the amount of water available for the tree to grow. You can plant a tree in a moist environment and tit will grow taller than the same species of tree in a dry climate.
The genetic make up of a tree will effect the growth of the tree and the height. In genetic programs trees are selected that are taller and straighter then others of the same species and these are used to collect seed to grow taller straighter trees for planting.
Some species of trees will be taller than other species.
The age of the tree as a tree gets older it can not transport the water to the top foliage and the height will stop or reduce.

Here is a web link that will describe some of this

http://lamar.colostate.edu/~mryan/Publications/Hydraulic%20Limits%20to%20Tree%20...