Not too many theives have one of these machines just laying around. Buying would probably cost them more than they'd get from any heist. And they aren't exactly "portable" in the sense of "throw one in your car and carry it inside". At least not the medical ones.

Besides, do X-Rays really penetrate metal all that well? I would expect most low power X-Rays to be blocked by the steel in the door of a safe. Maybe you might be able to see something if you cranked the power up high enough. But then, the thief would have just fried their gonads in that endeavor.

Just doesn't seem terribly feasible for your garden variety thief to go lugging an X-Ray machine around to their home breakins. Not really feasible for a high end thief either.