Let's face it, for an inexperienced parent there is no rational way to pick a pitching coach. Even if you're reasonably happy with your choice, you don't know if it was the best choice unless you shop around a bit. I wouldn't take that as an insult. If someone picked you first, they are happy with you because they don't know any better. If someone shopped around and came back to you, they made an informed decision that you were the best.
Also, there's more to it than just pitching technique. Maybe they hope for someone with a lower rate, or someone who gives lessons on Sunday morning, or maybe their TB coach pressured them to go to someone else.
I've had some leave because they want an indoor place because it's too hot to pitch outside.