Because they like you. They like you because you provide great value. You provide great value because you care about your clients, about being efficient and reducing costs, helping them achieve their personal and business goals, and you give them freebies (i.e. free advice occasionally, CLE, and more).
The result: raving fans! Raving fans are willing to pay more because they know, like and trust you ALOT.
True story: I once had a couple of independent auto mechanics who I was willing to pay more for a brake job, because I was convinced that they were honest and would take better care than the local dealer. I became a raving fan. It helped that they charged me nearly $200 less than the price they initially quoted me. After that I never asked again for a price before they worked on my car. Additionally, I sent them, by their estimate, approximately 20 customers over the next 8 years. I was a true raving fan!
What reminded me of that story was a post by Roy Ginsburg on Attorney at Work. He compares lawyers with barbers/hairdressers who charge more, but their customers are happy to do so because they like them and trust them.
His story is worth a read. Maybe you can get your clients to pay you more by turning them into raving fans.