Why can't the insurance cover my wls surgeon to take my gallbladder out?

I have medicare as my primary and medical as my secondary. I have an HMO under my medical. I had gastric bypass a year ago, but now it is time that I have my gallbladder removed.  I want the surgeon that did my rny to perform the surgery, but they said they can't because they are not contracted with the hmo for general suregery, but are for bariatrics. This makes no sense to me because if I have to find someone else to do it It could take up to a couple of months because of referrals. Is there any way to work around this so that I can have my rny do the operation?