The Independent Urologist says that physicians are at a disadvantage in contract negotiations with insurers:

There are simply too many of us in every specialty. In fact, this woman told me that the panels are over filled already and that she spends most of her time just getting new doctors accepted into the plans under any terms. While it is not impossible to get a better-than-average contract, it is difficult.

In major urban markets like NYC, this is difficult as there are many competing doctor groups.

Smaller areas may have more success. Combined with the fact that hospitals are buying more practices, physician leverage can increase under a larger umbrella.