It's not, but it's certainly a factor. EMU and DMUs are not manufactured on a regular basis in the US due to the different requirements and the size of the market, so each order is essentially an expensive custom order. The rolling stock that does exist here usually performs less well.

Because of the cost of (and in some cases impossibility of) buying multiple-units, passenger railroads here are operated using a locomotive and some railcars; this is very inefficient, especially for commuter or regional rail which stops and starts fairly regularly. DMUs would almost certainly be cheaper and more performant to run.