They were expecting big slow drones that would take hours flying from Ukraine. Instead they got many small ones that took 18 months to arrive but were invisible until the last minute. A surprise zerg rush, when they were expecting and preparing against ultralisks.

(My analogies are very weak today)

Pretty much all of their air defense seems to be stationed at the front and around their oil facilities.

I'm wondering if the man power drain is affecting them that much.