We can't really have a definitive answer to this until Putin and Shoigu publish their memoirs, or better--memos of their internal discussions, which however I don't think I'll live long enough to see (published).
A few more facts worth adding though:
The Russian leadership clearly doesn't want to risk its aircraft in the same way they are sending waves of tanks, APCs & IFVs packed with infantry at the Ukrainian lines.
They have other (unmanned) means for strategic bombardment of Ukraine that they used a fair bit: ballistic and cruise missiles, even in their crude Iranian-inspired variants. Strategic bombers have sometimes been used to launch missiles from afar.
Ukraine has also struck the bases of these bombers a couple of times, with various improvised drones IIRC, because they're far away and Western nations don't want to provide Ukraine with weapons to hit back far into [pre-war] Russia.
More recently Russia has been using their tactical manned aircraft to launch 'glide bombs', reportedly from as far as 70km from the targets. It's been said these have been a factor in their success in Avdiivka.
Western countries have sent a certain amount of their modern SAM systems to Ukraine. Just based on open sources, the exact impact of these is more difficult to judge on fixed-wing aircraft, but there certainly have been videos of Russian helicopters destroyed by these.
NATO shares with Ukraine real-time or "near real-time" radar data from their AWACS flying over NATO space. This reduces the exposure (time on) of Ukraine's own surveillance radars, making them more survivable.
(If there's some doubt on these statements, leave a comment on what you doubt, and I'll add some references.)