I will be very interested to learn the reasons behind this someday. As far as I am aware the Russian air force was supposed to be massively quantitatively and qualitatively superior to that of Ukraine but they are not operating that way.
I think it's probably a combo of a few different reasons:
1) They have maintenance issues like the rest of their equipment
2) I think they have weather related issues they don't want to admit
3) Stingers have had more of an impact than they thought
4) trouble finding "legit" targets that they can actually hit
The more we see, the more bush league their conventional forces look. Unless this is all some clever ruse, I have to say the PR nightmare of what in reality is the Russian teddy bear is going to be hard to shake.