Its political willpower

The US alone could hand over enough equipment to transition the entire UAF into a NATO standardized army.

To fully equip these slotted bridges with what would normally be a US ABCT the US has more than enough equipment to not seriously diminish its own fighting capacity.

It just comes down to handing over the equipment without removing the DU armor, delaying allied armor sales, cannabilize national guard units [30th Armored BCT (NC), 1st Armored BCT (MN), 155th Armored BCT (MS), 278th Armored Cav (TN), 81st Stryker BCT (WA), equipment (it already is doing that for the Bradley's)

Its theorized the reason ATACMS or cluster munitions hasn't been sent to the UAF is because the army never ordered new ones post Iraq War so the inventory is 'low' in the eyes of the army. ~1000 missiles is what it theorized to be. The other theory is they want to dangle it so the Russians don't get Iranian SRBMs. Either way, its political willpower on why they haven't been provided with it.

There are other arms that could be transferred that would severely assist the UAF in the coming offense like breaching equipment and lots of it. But the army doesn't want to part ways with it to again diminish its own warfighting capacity.

If Biden wanted to hand over enough equipment to the Ukrainians to fill out 5 bridges worth of equipment, the US military could realistically do it in less than 6 months if ordered. The only realistic hurdle would be having to have Congress authorize the transfer due to the valued amount of equipment and waive the DU armor being removed.