Because it confirmed to Putin that American president is weak and without principles. US just let everything invested into Afghanistan down the drain - there was a whole new generation who only knew a life in relative freedom. All that was lost. No, Biden did not start pulling out of Afghanistan but he displayed that US can accept disgrace, that they can be humiliated and they just take it.

The fact that Putin wants to annex Ukraine has been known for long time and it looked like a good moment when he could try to accomplish this.