From what I can gather from a week of obsessively following military experts’ explanations of the conflict, there seem to be two main factors at play.
One is the Russian choice of overall military tactics, which is derived from their political goals. While we can’t know exactly what it is, we can assume that Russia plans to impose some sort of political settlement on Ukraine (could be anything from puppet government to partition to annexation). Whatever this settlement is, Russia needs more than Zelenskyy’s (or some Quisling’s) signature on the dotted line: Russia very much needs at least the grudging acquiescence of the Ukrainian military, state apparatus, and general public. Without this acquiescence, Russia will be tied down indefinitely in counterinsurgency and “state building” with an unsustainable cost in manpower, money, and probably blood. To prevent this outcome, Russia initially tried to bypass most cities and focus on rapidly taking Kyiv and Kharkiv with minimal destruction. Hence the (initial) minimal aerial bombing and the various failed tactics such as yeeting paratroopers into airfields with no backup and sending light vehicles on Leroy Jenkins thunder runs into Kyiv and Kharkiv. Needless to say this plan did not succeed. So now Russia has a choice: ratchet up the destruction and try to grind Ukrainian defenses down, or fold and get out to avoid the long-term quagmire, which might eventually endanger Putin’s political survival at home. Problem is, folding would be a major show of weakness for Putin, which could also endanger his political survival at home. So far, we are beginning to see an intensification of shelling of cities, indicating that he is not folding. If this continues, Ukraine should expect to see more bombing from the Russian Air Force (coupled with more shelling from artillery).
The second factor seems to be the prevalence of portable air-defense systems (MANPADs, such as Stinger missiles) in the Ukrainian military. These systems are much harder to find and eliminate than larger anti-air weapons like the Buk or S-300, to say nothing of fixed-location air defense systems. MANPADs are also cheap—much much cheaper than military aircraft. So it becomes an expensive proposition to use aircraft at low altitude for missions like ground support. With MANPADs continuing to flow to Ukraine from friendly governments, this factor does not appear likely to change any time soon. So while Ukraine may see more high-altitude bombing as Putin intensifies the destruction, the Russian Air Force might remain leery of conducting some types of aerial operations.