I think its because dogs will do anything for their owner they are very loyal, but cats have a mind of their own and they will do what they want when they want and some people don't like that. and alot of people are allergic to them.
Dogs are way more interactive and affectionate and willing to please. Cats couldn't care less if you're here or not. Sure, once in awhile they purr, or jump around after a string, big deal. Dogs will run around with you and go swimming and get stuff. Cats, well, they just,,,,,,,.Dogs will protect you from harm. Cats say, "You're on your own here buddy, remember to feed me when it's over". Anyway, it all ends up being personal opinion. No right, no wrong.
Dogs are better at displaying and reciprocating affection. All cats do is meow, purr and rub up against your legs. Whereas dogs will do the tail wagging, jumping, licking, whining, and barking. Cats can also be snotty and stuck up, but I still like them.
You know, I think it's also a lifestyle thing. Dogs must be exercised daily, and get several outdoor potty breaks -- which doesn't fit everybody's schedule. However, it's interesting: It seems like more cat owners like dogs too (whether they currently have one or not) but fewer dog owners like cats, too. Maybe that's an illusion, I dunno. I do know that 86.4 million cats are owned in the US -- and only 78.2 million dogs are owned in the US. So, cats are way more popular than people think! :D