Hiring a "little person" actor is expensive, let alone the makeup job.
I heard on the radio a few weeks ago about how an English theatre production had saved money by having children play the parts previously given to midgets and having adults read out their lines over a microphone.
This, to me, makes more sense than any other explanation.If you take into account the differences in environmental needs - even comfortable temperature - it makes sense that most of the crew was one species.
Remember, there was one ship that was entirely crewed by Vulcans...so, there you go...
Agreed, pretty nifty solution to the whole thing. The Vulcan ship would be hotter and have thinner air I'm guessing.