This is not really about "willingness". Modern Western tanks are a logistical nightmare, and Ukrainian logistical capabilities to support them are limited. In order not to overextend them, everyone concentrates on doing what is most effective.

In the case of this war at this very moment, it is long range high precision artillery. HIMARS eats rockets like crazy, it needs to be supplied constantly.

Once those supply chains are fully built and safe, they can talk about adding tanks to the mix.