Cats are obligate carnivores, generally they will eat flesh and that's it. They are also usually smaller, a smaller body means running out of resources faster.

Cats are obligate carnivores, generally they will eat flesh and that's it. They are also usually smaller, a smaller body means running out of resources faster. Dogs are a bit more omnivorous than cats. So in the situation where they are trapped with the owner's body, a cat runs out of options faster than a dog. A dog will chow down on house plants, fruit.. or anything it can in extremis. A cat rather more quickly finds itself in the "eat this body or die" circumstance.