This is just my pet theory, but I think the problem is that Russia could not have actually been rapidly defeated. In order for a country to lose a war, they have to stop fighting. They have to surrender. Even had Ukraine retaken all its territory the first year, the Russians would not simply be willing to give up and call it a total loss. They would probably fight on. This would likely result in more Ukrainian attacks into internationally recognized Russian territory, which would further pressure Putin.

Facing that degree of danger, to himself personally as a strongman catastrophically failing to seem strong, he would really only have one option to save face–tactical nuclear weapons to try to re-aquire territory.

So, if preventing the use of nukes is a major goal, then the Russians need to be defeated more slowly, so that by the time the Ukrainians retake their borders, the Russians are more exhausted, with lower war support. This might bring them to the peace table, or they might fall apart. But they’re less likely to use tactical nukes to try to turn the war around and start winning again, in a process that would certainly take years.

Basically, the war was always going to take years, no ways to prevent that. So, it’s a question of what position you want the Russians to be in for the bulk of that whole duration of the war. And from the US perspective, the desired position is “no nukes needed”.

My theory anyway.