The best theory I have seen is the Biden administration is trying to ‘manage’ the conflict. A belief they can dictate levels of aid to determine a geo politically satisfactory outcome.

Concerns around unmanaged escalation made sense but the UK has been pretty focussed on methodically moving up the escalation ladder to demonstrate Russia won’t resort to nuclear strikes (Brimstone, Challenger 2 tanks, setting up the F16 coalition, Storm Shadow, etc…).

The USA expects China to be the next conflict zone, that is a naval/air situation where ATACM’s can’t be used. Suffering a shortage of ATACM’s in the near term isn’t really an issue especially if you’ve already put in place contracts to address the gap.