Nukes aren't that great a weapon to use against the country right on your border that you're trying to annex, especially if you've been busy justifying the invasion to your own population as liberation.
There are "tactical" nukes these days that can be "safely" used and in the beginning of the war, nobody was sure if the Russians were willing to use those. But even Hiroshima has recovered from a dirty nuke with relatively minor permanent losses.
