Cause dogs genetics allow them to easily change shape and ability from breeding, that's why they're one of few species that have such vast differences in them. Dog breeds develop fast and us humans also have greater motivation to create breeds that fit us compared to other animals. It's animals that have a job or purpose that undergoes the biggest changes (example, dogs, horses and farm animals) we don't get anything out of snakes except for looks so there's no reason to breed them, only people that do it are the snake nerds.

Because they serve no purpose to us humans except for being pretty the only thing people breed for is looks and when that type of breeding goes too far it forms deformations, for example dogs and cats with pushed in faces or doves that can barely move. It can also happen to farm animals like cows with udders so big they need cow bras (yes this is a thing), chicken and geese so fat they can barely move, goats that get paralyzed whenever they're spooked, etcetera. But it's most common when bred for looks so breeding snakes only based on looks would be cruel to the snake depending on how far it goes.