

Because Gorbachev fell for the liberal lie that if you just marketize and privatize everything, things will magically somehow work out…and Deng did not. In short, Gorbachev was an idealist moron and Deng was a dialectical materialist.
Also Gorbachev’s wrecking ball style economic reforms were accompanied by an equally sudden and disruptive political liberalization that was undertaken without any sort of plan or consideration of the consequences, again due to a naive belief in the liberal narrative that total free speech and free western-style elections would usher in a democratic utopia overnight.
Of course many in Gorbachev’s camp knew exactly what this would lead to and were planning on it, as their intention all along was to destroy socialism and establish themselves as the new ruling oligarchs. But we should also not discount the pure naivety, gullibility and stupidity of some of the people involved, including possibly Gorbachev himself, who had genuinely bought all of the West’s propaganda hook, line and sinker.
This is an excellent write-up, comrade!
The only little thing i would take issue with is that i think you’re being overly critical of what you call the “Stalin model”. Different circumstances call for different approaches, and i would argue that at that time, the USSR simply had to undertake rapid collectivization and industrialization in order to catch up. They could not afford to maintain the NEP any further as the development would have been too slow.
China’s model works great for building up productive forces provided you are in a period of peace and foreign countries are willing to come and invest capital. But the USSR was far more isolated globally in the 1920s and 30s than China was in the 80s and 90s. And war was on the horizon. If the USSR had adopted in the 1930s a model like that which Deng Xiaoping implemented, they would have lost the war against the Nazis. I think you recognize this as well:
And in the years immediately after the war they needed to rebuild a lot of what had been destroyed, they were in no position with much of the country still heavily affected by the war to go directly into such an ambitious transition as “Reform and Opening Up”. And then the Cold War started which meant that once again the USSR was under an existential threat and could not afford to shift to a more consumer economy, at least not until they developed the atom bomb.
But by the time that the situation had stabilized and they could have safely undertaken a Deng-style shift, Stalin was already dead, the principled Marxist-Leninists were sidelined, Khrushchevite revisionism had taken hold, and there was no political will anymore to take risks by radically reforming a system that had until then worked very well for the purpose of turning an underdeveloped, backward, agrarian society into an industrial superpower.