When it comes to game design of any type, the number of factions, the base that all other aspects can be taken from is an important decision.
Both race and class combinations give a lot of options, as well as storyline choices. Whether 2,3, or more. Rock paper scissors is a basic theory. Balance through Imbalance. In many games it comes down to chance and basic mathematical formula, but the challenge comes down to being able to overcome those odds. A chance, the one moment of legend when chance happens and one hero rises above the odds.
This is the moment in any story, any film, a well established story, structure and explanation that fully draws the reader/viewer/player into the moment, and everything clicks, the revelation of the tale, the moment of synergy when everything is in the hands of the Gods, and it works. Fewer races risks a simple story, many risks no balance in difference.