Simply put: Volcans did not have interest in space. For a society where greatest challenge, honour and reward are self-improvement and seeking perfection, service on a spacefaring vessel would be hindrance rather than help in attaining those goals. Any shipboard duty leaves rather small amount of free time... Just ask anybody with experience in navy.
And as always, there are exceptions... Obviously.
And no, Starfleet did not encourage homogenous crews (DS9 case of all-Vulcan ship was special dispensation IIRC), as is implied in other answer. I don't know if ENT finally made canon, but even with that discarded, it's clear that Federation came to be as a solution to one-race-crew-ships problems in the first place...
Somewhat different was diplomatic/political service - those offered better opportunities to perfect oneself (patience and smell, especially), and were much less time-intensive. So they could be viewed more favourably.
EDIT: Eating a frog here. It seems that canon indeed allows, even insists on segregated crews (from Gene Roddenberry himself: http://i.imgur.com/bDFzA.jpg?1). Shows how much problems creates sticking to the rules invented so long ago that no one remembers them...
