Yesterday, I had a post on how some cardiologists have reduced radiation exposures. The New York Times has a good background article on the broader issue, and the author explains some of the underlying psychology which has led to the increase in the exposures.
Patients getting a radiation-based diagnosis or treatment enjoy modern medicine and can't feel any DNA damage. The costs are usually covered by a third-party (insurer), so the patient is not very cost-of-test conscious. A doctor is focused on whatever brought the patient to him/her in the first place and also wants to utilize all tools at his/her disposal to avoid a malpractice suit. The doctor may also profit from more testing.
But as my earlier post discusses, the situation is improving.
The U.S. FDA is actively involved.