The Russians can’t spare troops for occupation of cities. Russia can’t afford to place entire cities on welfare when cut off from their economic regions. To feed Kherson was about as burdensome as feeding a million Russian soldiers.
The Russians need to supply their troops for a winter offensive. This means moving lots of fuel and ammo, fast.
that’s one explanation
another is that Russia is simply losing on the battlefield
all the evidence I’m seeing so far points to option (b)