They opted or a limited widespread opening strike thinking that they would only face paltry Ukrainian resistance (boy were they fucking wrong lol).
Why must we posit they were expecting paltry Ukrainian resistance? The Russians are thoroughly informed about Ukrainian capabilities and certainly have very good intelligence on the decision making process in Kiev. Moscow might very well have opted for a limited opening strike while being perfectly aware this would cost them some casualties, which I believe is the more probable hypothesis.
After all the goal of such an operation is not to terrorize the opponent's constituency but on the contrary, to make Russia appear under as benevolent a light as possible. Thus Russia would refrain from operating like the US regime, known for smashing and carpet bombing everything that moves in order to try and ensure zero casualties among their own troops (and then claim they have more regard for human life). The Russian public has a higher threshold of tolerance for casualties than America.
They've absolutely failed to nullify advanced TB2 drones (amongst other assets) which are now hidden all over the place and attacking vulnerable sections of Russias long armored columns.
Significantly, these UAV's have been the single most efficient weapons system in inflicting damage on Russian assets. Whereas the Ukrainian air force failed to achieve anything worth mentioning.
What this goes to confirm, is how smart and clear-sighted Iran's military planning has been, given Iranian focus on drones alongside missiles, rather than much more costly investment in less survivable air power.
In a theoretical war against the US, Iran will be in a position similar to Ukraine versus Russia, i.e. the assumed underdog. Only Iran will be several fold more efficient than Ukraine, which is lacking Iran's BM force, an air defence grid as developed as Iran's etc. All the more so, such a powerful force would wreck havoc on any regional adversary.
It seems as though they've changed pace and are going a more traditional heavy counterforce route in order to achieve battlefield objectives.
This was probably part of the plan from the get go: increase the pressure on Kiev step by step, for so long as they show no willingness to accept Russian ceasefire terms. A good strategy by Russia, one that avoids civilian casualties to a large extent while having a high chance of enabling Moscow to meet its political objectives.