
This only make the story and narrative more unfocused. That will sacrifice quality for the sake of a not meaningful variety.
I have a thread in the DAI forum why multi-race is a bad thing in BioWare games from a storytelling perspective.
Not to mention, heavily consider what you are asking. Quarians are stuck in their suits. They also have weak immune systems and are susceptible to diseases and basic injuries. How would that make any practical sense from a storytelling perspective? Not to mention, animations, armor variety, romance, and many other things would be severely limited... Think about a romance between two Quarians with their helmets on...
Also, bear in mind that Mass Effect is largely from the perspective of humanity. It is exploring this unique and vast galaxy of various cultures and how they impact humans as a result. The Shepard trilogy largely explored that in great depth. If BioWare was to have another race as the lead, the back story, culture, and character development would have to be just as elaborate and compelling as the human perspective.
Part of the reason why games focus on humans is because they are obviously the most relatable, thus the most compelling. Again, ask yourself how practical it really is to play as other races and not see it severely hamper the experience.