In my amateur opinion, mostly SAMs plus proliferation of stand off weapons and general Russian lack of giving a shit about accuracy contributing to lack of russian air. Additionally, the longer the war the higher the number of complex Russian assets down for maintenance.
I know it's been said many times, but the Ukr Air Force still operating is incredible. Yesterday SU-29 taking down a cruise missile was great to see. Their distributed operations, maintenance teams, logistics, etc deserves tons of appreciation and study post war.
I am not so sure Putin can use the tired and true war of attrition, specifically with manpower. Most reports I've seen have the Ukrainians with a larger standing army in the war than the Russians. Russia moved to a command style war economy this week and using financial incentives/PMCs for shadow mobilization.
The Russian advantage in artillery, both in systems and ammunition, is their primary means of advance. Someone correct me if I am wrong, but the Russians still have enormous amounts of both, their weakness is the logistics.