I would say it is a general fear factor of the "unknown"...things like:
1. How do I know the billing company is worthy, and will do their job right, and not take shortcuts like writing off denied claims, or just drop the ball completely and not file claims on time?
2. Will we have to switch software and why should we, and how badly will that disrupt the office/staff?(As Linda's post talked about)
3. How much is this really going to cost me/actually benefit me?
4. What are the risks? Will my income be affected negatively by making the switch?
Doctors, as with most people, don't like change. They don't know what to expect with a billing company, and many have heard horror stories from other doctors who signed on with a less than great company, and paid the price for that decision. Moving the bill out of house requires giving up a certain amount of control, and giving that control to someone else they don't necessarily know, and that is scary. Even if the doctor may not really be on top of what their in house biller is doing(who could be the worst biller ever), and could actually do much better with a billing company, they are still leery of the process. Change is hard.
I liken it in my mind to someone who is afraid to fly. I was one of those people, and didn't fly for years. I spent ridiculous amounts of money driving everywhere, because I am a control freak, and in flying, had to give that control up to someone who was qualified to fly the plane, and that someone wasn't me. It didn't matter that flying was cheaper, faster, safer...I didn't understand how it worked, and I wasn't in control. So what I did was educate myself on planes, and what could go wrong, and how my fear of the unknown/loss of control was hindering my life. Now I fly every other month, and while I don't "love" it, I realize that I am in the hands of a highly trained set of professionals, my life has become much easier, I get results faster, and I am saving a ton of money.
While this isn't a perfect analogy, I think it works, somewhat. The doctor can sit in the driver's seat feeling like they are in control, and waste a lot of time and money having their staff do the billing, or they can turn it over to someone who can do it faster, cheaper, with better training/results, usually. The key is educating the provider on the process, and how the benefits far outweigh the "potential" negatives.