Because that part of it is on western half of it on other side of the Dnieper river and Russia doesn't want to lose troops and waste resources defending it.
Russia has already liberated and taken all the land the vast majority of land it wanted. Clearing out what is left of Donetsk and Lugansk is all that is left to do.
Building a land bridge between Crimea and mainland Russia and taking control of most of the Black Sea coastline was the objective behind liberating Donetsk and Lugansk. And, that has been accomplished for well over a year.
Looks like you went back to try to edit it, and replace "all" with "vast majority" but even that is wrong.
They clearly wanted all of Ukraine, but now probably won't even get the full four provinces that supposedly succeeded.