People with smaller/medium businesses don't typically put a lot of emphasis on websites (based on my measure/references). Users of these services probably acknowledge that fact and have grown accustomed to scheduling appointments by phone.

On the other hand, small/medium businesses that do put a large emphasis on their website, and do have a client base that regularly utilizes their online presence, might have anxiety about passing their clients along to another site to place appointments. They may or may not have their own online solution for this. But regarless, I'd bet they want to own that function themselves. Do you offer your service as module/plugin that they may insert into their site easily?

Another thing I'd suggest is being more clear on your home page about what you actually do (as a software). The testimonial on the home page is great, but not the best place for it.