One hypothesis making the rounds on Twitter is that Putin’s bombing campaign was targeting US funded biolabs in Ukraine [0][1] which the Russians have previously complained about [2]
I agree, bombing labs from the air doesn't warrant boots on the ground, so it's obviously not the full explanation. I shared the hypothesis because grandparent comment asked what other reason there could be - this was one.
[0] https://twitter.com/Manifestor4/status/1497162095762288672/p...
[1] https://twitter.com/jqkennedy17/status/1496793895040008192/p...
[2] https://www.newsweek.com/russia-china-nikolai-patrushev-vlad...