I haven't gotten involved with any roleplays myself in game, though I have a sneaking suspicion that the reasoning behind having a few of these "Wolf only" or "Lion only" RPs is to sniff out mates. With a younger crowd that has started to figure out that "FEMALE LION LOOKING FOR MATE!!!" isn't going to result in anything, this is their next best bet.
Now, I'm sure if you found an older, more literate group of people, they would be more likely to consider a multi-species rp.
But this is just a guess ;P