A couple of factors favor the AMCs ...
They can negotiate better rates with payers because they're large. In essence, they bill and collect more than the anesthesia group can, for doing the same work.
Some are backed by venture capital, and don't necessarily expect to be profitable during their growth and acquisition phases. There's a long and glorious history in every industry, of organizations with deep pockets willingly taking a loss while undercutting the little guy to drive him out of business.
Bottom line: It's hard to compete with a company that gets paid more than you do for the same work, and that doesn't even have to be in the black to stay in business.