>>17915930
You might find Chinese takes on it interesting because they have a Leninist party structure there and they've also put some thought into it. Here's from their (state-controlled or at least monitored) online encyclopedia.

Basically the main problem as they see it was political, then economic. The party-state was excessively centralized during Stalin's time, which bred corruption and formed by a new privileged class by Brezhnev's time (but that's when the USSR had a really bad falling out with China so keep that in mind when they are more critical of Brezhnev than Stalin) which became detached from the population they ruled over. It was too rigid (politically and economically) and by the time they began to make reforms, it was too late, and they screwed it up.

Then going down the list, other factors included the excessive militarization to compete with the U.S. for global hegemony and which consumed a lot of resources. It also says that Western propaganda played a role, but that was not a primary reason (as it's only effective in proportion to the amount of internal problems in the society... that's a very Maoist notion and the line to describe this paraphrases Mao actually, and I also think it happens to be correct). Then lastly were ethnic conflicts as "the Soviet Union's long-standing application of Great Russian nationalism resulted in rather sharp contradictions between the various ethnic groups."

The USSR basically turned into an oligarchy ruled by a privileged class who gave zero fucks about most of the population, and they had no outside feedback or checks into their own system, while ruling over a rigid economy based on heavy industry and oil export earnings. The people tasked with delivering reports were the same people who were tasked with carrying out plans. You see how that works.