The reasoning is that Putin is trying to bring the war home to Russians and frame it as an existential risk. They assess it as unlikely that Ukraine could penetrate the air defense around the Kremlin with drones.
In previous instances of actual Ukrainian successes like the retaking of Kherson, the Russian response has been far less organized and timely.
Russia may use the attack as an excuse to cancel or curtail the May 9th Victory Day holiday. And to set the stage for further mobilizations.