Bryan Caplan argues that the Soviet Union fell because of a shortage of evil. In his blog posting Evil Shortage: Why the Evil Empire Fell Caplan writes,
So why did the USSR fall? Because a new generation of half-hearted Communists like Gorbachev were allowed to take the reins - and they just weren't evil enough to retain power. If Stalin had been in Gorby's shoes, he would have reinforced the foundations of totalitarianism by murdering a few million people - beginning with active dissidents and gradually expanding to anyone who'd ever visited the West. And like Kim Jong Il, he would still be in power today. Gorbachev's generation of leaders were far from saints, but as tyrants go, they were wimps.Of course it could have been that the economy had turned to crap and just couldn't keep on working.