Why do populations evolve and become adapted to the environment?

The short, simple, answer, is survival. The environment is one of the things that drive evolution. When a species is suited to the environment in which it lives it remains relatively stable in it's particular form and exhibits little change.

If the environment becomes unstable or is radically changed in some way, such as ever more cooling temperatures, or a depletion in some natural resource, this drives a species to adapt to the new environment through change/evolution. Depending on the speed with which an environment can change, or the severity of the change in the environment, the pace of the evolution can be relatively slow or relatively fast, but in geologic terms, either fast or slow evolutionarily speaking, the change is still slow in human terms. Evolution always takes usually several millennia, and on rare occasions, based on the scope of the environmental changes, it can happen in as little as several centuries, but this is the exception and not the rule.