Russia needs to reserve its supply for all of the lucrative trade deals it's about to make with the US.

So the reason is that they can't deliver gas beyond the first of January anyway due to Ukraine shutting down the pipeline, and due to unfulfilled obligations they wouldn't receive payments anyway, so they can shut the delivery down right now. And politically, they obviously see no harm in burning even the last few bridges into the West. Or maybe Gazprom is so broke, they can't afford it even if they wanted to.