It would take a book or more to really explain what went wrong. I'll try to tl:dr, but it'll still be a little on the long side.
Marx did not leave behind any blueprint on what communism needed to be. Nor did he leave any road map on how to get there. So right from the start there were too many missing pieces. And many of the pieces they did have were wrong. Particularly in not understanding the roles of money, prices, markets, and services in the economy. Simply put, there was too much focus on physical goods. And too much assumption of communism arising in an industrialized nation among an industrial proletariat. Guess what? 1918 Russia was not an industrialized, or really even an industrializing, nation. It was a peasant agricultural nation not even fully removed from outright serfdom.
So you kill the czar and his ministers, and as many of the other nobles you can catch, and what have you got left? Peasant farmers. Peasant farmers was not what Marx was talking about for a communist revolution. What's a Lenin or Stalin to do?
I'll fast forward past most of the forced industrialization to after WWII. This, for the point of our narrative, is where Soviet development policy really started to go wrong. You see, the Soviets could, and did, copy a lot of technology from the industrialized powers. This is a latecomers advantage that many nations have used. China has been using it for the past 20 years or so. What it means is that you don't have to reinvent the wheel, if you can see an actual wheel, see it in use, see how it is made, and so forth. There is no need to invent and implement each of the intermediary steps. This saves tons of time and effort and investment. So the 1940s and 50s were times of really great growth for the USSR. They went all the way from 17th century peasant agriculture to 1940s industrialization in the 4 decades after the revolution. That's a huge explosion in labor productivity.
And then they more or less just stopped.
You can have the best 1940s era tractor factory in the world, but if it's 1970, then it's obsolete. This is where the theories of Joseph Schumpeter show the difference. Creative destruction means that when you create something new, something old is displaced by it. The old is destroyed by the new. This applies to both product and process! In market economy nations labor productivity continued to grow. Because the factories and the products they produced were continually improved. In the USSR, they had a working tractor factory, job done. A man could get out of the Red Army in 1945 and go to work in a tractor factory and in 1975 he would be doing exactly the same job in exactly the same factory with exactly the same tools producing exactly the same product. Except that now all of them are old and worn out.
And that is why that once the Soviets had gotten everyone off the farm and into the factories that they could, there was no more growth to be had.
Now to add to their problems, they never did get the kind of agricultural productivity that Western nations did. And so they couldn't get as many workers off the farms. And they never did fully develop their transportation infrastructure, so they didn't even get the food that they did grow to market. In the 1970s the USSR, the nation with had the largest amount of premier grain growing area in the world, was dependent on grain imports from the United States.
So their agricultural productivity was stagnated. Their industrial productivity was stagnated. Their transportation was underdeveloped. Their services were underdeveloped. The why of it all was a leadership which was ideological in the beginning, but just became more conservative and corrupt with time. They did not continue to develop. Development is a permanent process. It is always ongoing. And they just never got that.