It's been that way for a while. The wars in former Yugoslavia in the 90s demonstrated this, as well.
Ukraine's population is 70% urban. Taking large chunks of countryside means very little in a tactical sense. On the other hand, tanks do little in urban warfare. The 50km column of Russian armour going towards Kiev looked impressive, but what were they going to do? Frontal assault on a city of 3 million?
Mariupol may well fall to Russia in the coming weeks, but if it does, it will be through surrender. I don't think the Russian army has the military capability or the stomach to take the city building by building. The casualties would be astronomical. You can use technology and pummel a city into rubble, but eventually real live human beings are needed to go in there and take it.