There is a significant difference between Medicare (Part A and B) and Medicare Advantage (Part C) plans. When you chose an Advantage plan, you 'lock' yourself into the health care plan of the insurance company you pay your premiums to. The insurance company (Cigna, BCBS, United Healthcare) dictates your coverage, network providers and payments to those providers.
Medicare Advantage (Part C) plans do not include Mayo Clinic as a network provider. Individuals with just Medicare Part A and Part B are free to use any provider including Mayo Clinic and are not subject to 'network' restrictions. I hope this helps you understand why you're unable to be seen at Mayo Clinic.