(Some typos in my original but I think you got it) In short they are up against "winter rains" - but it's end of winter rains. The rainy season in Ukraine begins in March and continues through August. So there is this short window in late winter to roll tanks through with true freedom of movement. The wet ground has frozen and hardened enough by February after several months of dry cold to support armor, but by the end of March enough rain has fallen to thaw the ground and bring back the mud. It's why last year right before the invasion every news channel predicted an invasion "any day now" and acted like there was some intel leak or genius breach into Russian plans. Not really, there's just only so much window to get your tanks anywhere with full freedom of movement.