Story. Russia is just not as strong as everyone thought.
No, Russia is quite strong.
People didn't realize 2 things. Modern armies are too tiny to fight major countries and win easily. The last big wars that happened during the cold war and ww2 had millions of soldiers. Russia only started with 150k. All Ukraine had to do was mobilize its reserves and Russia was outnumbered.
Secondly, drone warfare has made armored warfare completely useless. In the past the expectation was, armored maneuver warfare was the way to go. Now it's back ww1 style grind.
If you compare Russian strength in terms of military production, force reconstitution and economic resilience against western sanctions, its pretty obviously no one in Europe or western vassals like Japan or Korea can come close to this level of military strength.
Only China, US and Russia are on this level.