They are simply more metal.
Cats are totally metal. There is nothing more desirable than being metal—as it is the essence of greatness. Dogs are for people who like crap music and are substandard people in every way. Dog people like middle-of-the-road AOR, are overly conformist, needing an animal that boosts their social capital and for people who can't manage without constant love and adoration. Cats are for people who are individuals, who understand the majesty of a lightning fast guitar solo and can appreciate music at breakneck speed or able to slow down to a dirge. Cats are for people who give zero fucks about having an animal that they can dress up and teach to do tricks. It's for people who like an animal metal enough to simply kickass by existing (and pooping in a box).
ETA: Courtesy of Teapot: Metal Musicians and Cats!