And the problem with pepper spray for a truly savage dog is?
Slower to deploy. Good for raids when you know you're going to a place that might have dogs, and thus you can have it out, maybe even have a dedicated officer to take care of the dogs.
Not so good when you're responding to a domestic battery report. Domestic violence calls are the most dangerous calls a cop has to handle. More cops injured on them than any other sort. But, also by their nature, you generally don't want to go up with gun drawn, nor OC spray either.