I actually think it's elasticity of demand for imaging. Essentially every market force in the past decade that could have benefitted radiology has benefitted them - consolidation and corporatization of healthcare delivery, rise of midlevels, lack of transition from FFS reimbursement model, adoption of patient satisfaction metrics, lack of tort reform. All these factors heavily increase demand for imaging.
Half the imaging I order are useless diagnostically... yet I order it. Why? Because why not? The decision fork here is wildly asymmetric. If I don't order it, then my patient satisfaction scores go down and it would involve a 15 min argument with the patient about WHY imaging is unnecessary. Also, it covers my butt on the off chance that I was wrong. Given the FFS model, the hospital gets paid, so administrators won't send me emails about resource utilization.
However, the macroeconomics of healthcare and society in general are changing rapidly. It's likely that any of all of these forces will change in the upcoming decades.