The other responses you're getting seem to be getting it half right. The wikipedia article explains it pretty well: Low-Background Steel

The process of making steel requires using air from the atmosphere, which has small amounts of radionuclides in it. So if you're trying to make something that detects radiation, and has steel, you'd have problems.

The levels are also a lot less nowadays than they were when they were at their peak, meaning using modern steel usually isn't an issue for most applications:

World anthropogenic background radiation levels peaked at 0.11 mSv/yr above natural levels in 1963, the year that the Partial Nuclear Test Ban Treaty was enacted. Since then, anthropogenic background radiation has decreased to 0.005 mSv/yr above natural levels. This is low enough that, in 2010, Straight Dope said that "reduced radioactive dust plus sophisticated instrumentation that corrects for background radiation means new steel can now be used in most cases"