Part of it is simply that cats are less variable. Sure, they are different coloured or patterned or long or short haired, but the variation in dogs in size and shape is tremendous. Dog lovers typically like to represent their favourite breed, and you would need 100+ varieties of sweaters to make enough choices available. Cats? Well, even if it doesn't look exactly like your own cat, a housecat is pretty easily translated onto anything.
But like Angie mentioned, I have seen bulldogs and schnauzers (and owls!) on quite a few sweaters!