How does darwin's theory of evolution by natural selection explain why some organisms become extinct?

Answer: Species become extinct because they can no longer survive and reproduce in their altered environment.

Explanation:

Natural selection is an evolutionary phenomenon defined as the differential reproduction of the genotypes of a biological population. The conditions of an environment favour or hinder, that is, they select the reproduction of living organisms according to their peculiarities. Natural selection was proposed by Darwin as a means to explain biological evolution. The trait subject to selection must be heritable, there should be variability of trait among individuals in a population and that trait variability should result in differences in survival or reproductive success, making some newly-appearing characteristics extend into the population. The accumulation of these changes over generations would produce all evolutionary phenomena.

Species become extinct because they are not able to survive and reproduce in the environment. An members of that population cannot adjust to change, they do not adapt.