There aren't many cases where having an X-ray done a little faster would be massively helpful. Emergency department physicians and trauma surgeons can read an X-ray for acute trauma in a matter of seconds, and doing the X-ray itself doesn't take more than a few seconds either. It's hard to think of a pathology that would show up on X-ray but not through the patient's presentation or physical exam. Going off memory, I can't think of a situation where an X-ray would be the crossroads that determines what I do next for the patient. Also, an X-ray done in a moving ambulance is probably going to be of fairly low quality. In most cases, they'd just have to redo it in the hospital.

The mobile CT scanner is also not super helpful for much other than determining whether a potential stroke patient has a grossly hemorrhagic stroke. In that one case, it is beneficial, but I don't think mobile CT scanners are used for much else.