The fruit fly Drosophila melanogaster was found to be very suitable for experimental verification of chromosomal theory of inheritance by Morgan and his colleagues because

Morgan worked with the tiny fruit fly, Drosophila melanogaster. They were found very suitable for such studies.

They complete their life cycle in about two weeks, and a single mating can produce numerous progeny flies. They could be grown in a simple synthetic medium in the laboratory.  

Also, there was a clear differentiation of the sexes – the male and female flies are easily distinguishable. It has many types of hereditary variations that can be seen with low power microscopes.