Why always (afaict) bulldogs? I’m thinking maybe their lower center of gravity helps. Do the dogs actually steer and consciously go where they want?
I met in person the dog that I think was the first of them, and he was a bulldog. That might have prompted imitators, and bulldog-owners might have been more prone to train their dogs to imitate him.
For what it’s worth, when he found himself going in a different direction than he wanted, he would dismount, get back on the other way, and go in the opposite direction.
Our neighbor’s dog can ride a skateboard and even had a short clip of it shown on America’s Funniest Home Videos about three years ago. Half Jack Russel terrier, half beagle. She’s smart as a whip too. I often think of her as Snoopy’s granddaughter. (The dog, not the neighbor.) Doesn’t steer though.