I'm quite surprised smaller cetecean species aren't popular in captivity, they would solve some of the bigger issues with the larger species such as space.
Yeah, I'm also surprised that there hasn't been as much attempt to branch out into other cetacean species. A number of captive cetacean places are heavily focused on entertainment, (I'm talking about theme parks, tourist attractions like resorts, that sort of stuff) so it makes sense that they'd stick with the popular (and more trainable) animals, but I am a bit surprised that zoos and aquariums haven't experimented more. (and not just for space and care, but for the sake of species diversity) I suppose the presence of the entertainment facilities creates a more easily accessible stock for zoos and aquariums, so the entertainment-oriented facilities are going to influence what other places have.
Plus, a lot of the smaller species tend to be rarer or more elusive in the wild, making them less prone to capture and less likely to end up in rescue-and-keep situations. I've heard that most porpoise species have proven difficult to keep in captivity, but you guys can correct me if I'm wrong.