Why do species evolve during adaptive radiation to fill different niches to create variation to become reproductively isolated to provide evidence of evolution

The answer is to fill different niches.

Adaptive radiation is the process of development of new forms of organisms from an ancestral species. When an environment changes or some new niches open, speciation from the ancestral species occurs. New species have morphological and physiological adaptations which allow them to exploit a whole range of divergent environment.