Russia had an absolute massive stockpile of air defense assets at the beginning of the war. Ukraine has steadily worked at degrading these assets over time. The more they destroyed the easier it was to destroy the next asset.

At the same time Ukraine has massively increased it's drone capabilities and numbers at every range. So the remaining Russian air defense has to work harder with less numbers against more drones.

The longer ranges of Ukraines drones also means that Russia has to defend more area with less equipment. Russia is forced to prioritize assets to protect.

Today Moscow has a robust air defense system but whole sections of the front have close to zero. Ukraine has total freedom to hit any target they can find in many regions.

Russia is seeking to figure out a cheap effective way to defend their airspace. At the same time Ukraine is relentlessly damaging the infrastructure Russia needs to build the new anti-air capabilities.