Because Ukraine is losing, and particularly losing the larger caliber guns that can penetrate Russian armor. So NATO, that famous "defensive organisation" will escalate, again, to using weapons the world, including NATO, has said should not be used. It is hoped the DU rounds will make up for the lack of better guns.

This is "the rules based order" of the world today. The rules are whatever we (the US, and also our vassals and proxies) say they are. International law? Ha! The ICC? Nope, we'll invade the Netherlands before we follow international law.

It is not a surprise that in the decades since we invented "the rules based order" our enemies and adversaries continue to grow. Everyone sees us for the hypocrites and bullies that we are.