Aside from the bigger companies, I can see why some of the smaller ones wouldn't want to do this, like the restaurants or his vet. Most of the time they just hired someone to create the website, and be done with it. They don't know how to implement the accessibility functionality themselves and hiring someone to do it may cost several hundred to thousands of dollars.

I was interning in a government related web development job, and it took several weeks to properly remake a site to conform to WCAG AA level standards. Unfortunately, that was just for the templates. The dynamic content generated by the CMS usually breaks the conformance. Making websites accessible is not an easy task, and could cost a heavy amount for small businesses.