It's not just minefields, but also how easy it is to deploy more minefields. What's behind the minefields they need to penetrate? More minefields.

The Russians just got a lot of time to fortify, and resolved a few of the problems that struck them during the last counteroffensive. They do learn from their mistakes, they've always been "not bad" at that, on the battlefield anyway.

Blitzkrieg won't work again, and the Ukrainians probably guessed that. Fortunately, they're getting enough tools to prepare other strategies. Air power opens a lot of doors.