Because the State Department and WH plan for the war was going so well.......
The Russians had found a "successful strategy" (If you don't give a sh*t about your troops). Spend your troops to find the fortifications and glide bomb them out of existence, rinse and repeat. Russia I suspect figures they got the troops and AFV for two more years of that type of fighting, which will grind down the Ukrainians and force them backwards. The Russians likley figured that the West or Ukraine would settle on a poor deal before that point.
Ukraine could see this too and decided to toss a wild card in with the Kursk incursion. If Russia cannot drive them out quickly, it's going to change the game and force the Russian to spend resources there and to increase the defences on the rest of the border. The only benefit for Russia is that they now can lawfully send the conscripts to fight in the Kursk region, but at the same time, they can't afford to lose to many of them, for political stability reasons. It's impossible to say how this failure to protect Russia proper will cost Putin, but I bet he now sleeps with both eyes open.