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.