You know what? Every dog in the my city is required to be licensed and microchipped (can't get the license without a chip).

You know what else? Nobody cares. Very few owners comply.

Licensing is easy. Get your rabies shot. Send in a small amount of money for an altered dog or a large amount of money for an intact dog, and you have your license.

Forcing people to fix their dogs is much harder. So if licensing failed, why shouldn't mandatory s/n?