I think part of it may be that the first releases were designed to be much simpler monsters. First wave Clawdeen didn't even have clawed hands at the time, instead they used the more human hand mold. As Monster High has gone on, the designs and molds have slowly gotten more and more complex. I think with the tails, it would be too big of a change from the original design for them to do, but I have wondered before why the werecats have them and not the werewolves.