Originally Posted by kevin
Given how long lived the symbiont's are you might suspect that they don't breed frequently or in large numbers. Otherwise the Trill society would need more hosts.
Yeah, I think it's been suggested that there aren't that many symbionts and that joined Trills were rare. It's possible that the symbionts are an endangered or dying species and that joining with a humanoid host was the best way for their species to survive.