We no longer have the ability to manufacture new low-background steel on this planet due to environmental contamination.
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.
"... 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."