It’s also due to increased efforts to get people to spay and neuter their pets, including reduced cost and free sterilization programs.

I don’t think that’s it actually. There are fewer animals abandoned because of greater awareness of spaying and neutering and also more charities dedicated to moving pets around the USA. Traditionally, a lot of animals were killed down south, where strays are more of an issue. Now people pay to ship the animals up north where there’s demand.