Orcas are the only species in the genus "orcinus" and the largest species in the dolphin family.
They are probably descended from carnivorous land animals called mesonychids. They looked like wolves but their feet were ungulates. They went deeper into water in search of food and over millions of years developed fins, lost their fur and their teeth modified.
There is no real explanation, but they most likely evolved for survival purposes like new food and environments as well as to give them more flexibility in habitats
Most of the reason, is because of their differences in habitat, and food availability options.