Because after bribing Trump with the ability to brag that he won the USA 500B of minerals, they will be able to march in without any US interference...
Why would that change if he hasn't?
They have their land bridge to Crimea now, and if I had to speculate, they’d be happy with a neutered neighbor that can’t join NATO, essentially a populated DMZ. I can’t see what benefit in wanting to take Ukraine on again after the dragged out meat grinder it was this time around.
Expanded access to the Black Sea and natural gas/minerals were and still are very important to Russia. Aside from these, a total victory would allow Putin to cement himself as a conqueror in Russian history books.