There are several reasons I can think of:
- It was the first socialist country in the world, it was bound to make mistakes. All other countries that came after the USSR can study it and correct its mistakes.
- When the first 5 year plan was initiated, the Bolshevik party was desperate because they knew Germany was going to invade them sooner or later. This led to the censorship of all ideas that were different than rapid industrialization of the country. This censorship didn't end after WW2, and this opened the door for opportunists like Khrushchev to begin the de-Stalinization of the country.
- The Soviet electoral system was bad, and led to a systematic increase in abstention. This allowed corruption to permeate into the political class. Corruption is in a lot of cases a lack of democratic participation from the masses. Xi Jinping didn't fight corruption by simply executing billionaires, he increased the participation that the masses have in politics. This is why the approval of the local governments has been increasing.
- The population was not educated in Marxism.
- They overcommitted with military spending. This meant that the quality of life of their citizens became stagnant.
- They got baited by the West in regards to consumerism. Like not having jeans in the USSR was seen as oppressive and a sign that Socialism can't give people good products.
Eventually this led to a general perception that the politicians were corrupt, a romanticized view of the West, a government that was disconnected from the masses and a general discontent from the population regarding their living standards and consumer goods.