These machines are already expensive, adding a battery wont hurt anything else but profits.

There's also the matter of risk. If your machine that includes battery power malfunctions, you are liable. If the battery powered machine is separate, it's someone else's liability.

And liability is the driving force of most American healthcare. Don't believe me? The only reason why some vaccines are available in the US is that in 1986 congress removed tort cases involving vaccines from the regular court system. (And with the rise of the anti-vaccination movement, I think we can all see why that "some", could very well be "all".)