Is new mining of similar materials environmentally friendly?
"... background radiation has decreased to very near natural levels ... low-background steel no longer necessary for most radiation-sensitive applications,... However, some demand remains for the most radiation-sensitive applications, such as Geiger counters and sensing equipment aboard spacecraft. For the most demanding applications even low-background steel can be too radioactive and other materials like high purity copper may be used."
Basically we'd need to filter the gases injected to the process to guard against contaminants, which would cost a lot. So it's just cheaper to recycle old steel.