It’s not so much they want steel from before WWII, but they want steel that was produced prior to the detonation of nuclear weapons.

Because of the many nuclear tests conducted around the end of WWII, the level of background radiation in the air has increased. This increase is tiny, has no harmful effects, and is essentially unnoticeable. However, it is noticeable if you’re trying to create instruments to measure radioactivity (e.g. Geiger counters, etc.)

Because of the process used to create steel, any steel produced after the widespread detonation of nuclear weapons contains trace amounts of radionuclides- this can be an issue if you’re trying to make very precise radiation detectors.

However, if you are able to get your hands on steel produced earlier, before radionuclides were as present in the air, you are able to create more precise instruments. This is where salvage divers come in.

Wrecks of ships manufactured before WWII have MASSIVE quantities of steel that aren’t contaminated with trace amounts of radionuclides (since the ships were produced before the widespread detonation of nuclear weapons). This means they are highly prized for their use in making precise radiation detectors.