Ukraine is limited in its use of American weapons. It can't strike well in Russian territory. The US wanted to keep the conflict from escalating beyond Ukraine's borders.
Europe has similar concerns, but if Ukraine loses American support, they may not be able to afford that. They might gamble on direct strikes on Moscow, or at least make it clear that they can.
Which might end life as we know it. Or it might force Russia to the bargaining table directly with Ukraine. Who knows? Let's find out!
I am curious about this. Why you say it?