In reply to goldmember:
Tens of thousands of years of selective breeding of dogs for different purposes. Admittedly, I'm not sure what the purpose of a chihuahua is, other than to snap, cower and yap.
Cats don't have that long a symbiotic relationship with humans, because, as Maisie points out, they really couldn't give a shit about you, and aren't amenable to training. Pest control for agrarians, perhaps, but Jack Russells are far superior ratters, verging on psychotic killing machines...