Ukraine's home field advantage.

These poor saps are going to Ukraine with like ten days of training and rusted Kalashnikovs. If Russia's combat-ready forces couldn't handle the Ukrainian military, these guys aren't going to fare any better with less combat experience and training.

Russia can't just replenish the devastating losses suffered by the VDV, Marines, and Special operations forces.

Russia GDP is 10× Ukraine GDP but that's apparently not enough. Ukraine's superior military morale and technology outweigh Russia's economic and numerical military advantage.

Russia failed in Afghanistan too but everyone fails an invasion of Afghanistan. Where have they succeeded? Chechnya and Crimea. They bit off more than they can chew with Ukraine. I hope they will not escalate in desperation.

US GDP was 30× Iraq in 1990 but Bush did not choose to invade then. They waited until 2002 after weakening Iraq with a decade of sanctions and US GDP was 300× Iraq GDP. Never mind Afghanistan. And it still wasn't a complete cakewalk.

Home field advantage.