The same reason most of NATO have been very hesitant and the like:
Supporting a defensive war is one thing. Supporting an offensive war, against a nuclear power that threatens to nuke people on days ending in 'y', is another. And while it is incredibly unlikely that putin would actually attack anyone (since they can't even handle a Ukraine with one arm tied behind its back), it will still lead to political turmoil as people insist the world is about to end.
But now? This is a REAL good way to distract people from the other, much less defensive, war that we are financing.