I'm guessing cats are doing much better today than in previous polls is because of the fact that cats are low maintenance pets who can at least take care of themselves for short periods of time when you aren't around. Dogs simply get too attached to their owners to be left alone for any extended period of time. And with the aging userbase of this site comes more responsibilities like family things such as having children (and going out more often with your children), or caring for elderly relatives, which would create a conflict in being around often enough to care for a dog.