Why did Communism inevitably lead to dictatorship and totalitarianism?

Imagine someone has a great idea for a machine. They need someone to produce it, someone to organize the production, someone to set up the distribution, someone to sell the product, etc.

Every level requires a different skill and a different type of person to deal with it. When the money comes in and the company expands it will turn into a form of Communism. It won’t need many creative people, but more for production, and even more for all the other jobs. At some point, the infrastructure required to run the company will eventually be bigger than the actual production infrastructure and will continue to grow to the point where those running the company are more concerned with the company itself rather than the customers it serves.

That is, the company’s needs take priority over the customer's needs, and those running the company become concerned with self-preservation. At that point, the company becomes too big to change fast enough to be competitive.

Political systems become out of date, but those in charge will not relinquish their power, and so by attrition, the system either gradually gets undermined, or a revolt gets rid of it. And the process starts again. 

As the declared purpose of any political system is to distribute resources to the most people for the most good, it can only work if there are enough resources to be distributed, and the means of distributing them.

The aim of communism is to create a 'classless' society where everybody is equal. Karl Marx in his, 'Communist Manifesto' clearly mentions the process to achieve a perfect communist society.

Socialism is a road to Communism. Karl Marx himself advocated the 'dictatorship of the proletariat. This means the only single communist party which represents the working class. The private ownership of the goods is strictly prohibited and collective ownership is encouraged. The government basically controls all means of production and decides consumption patterns.

To achieve this you definitely need a highly centralized and dictatorial government as you are handling everything from paints to nuclear weapons.

Therefore all formally hardcore socialist countries have only one communist party and politburo or senior leadership 'approves' all nominations in the elections.

Communism was based on a beautiful and attractive Utopia. All states who committed to Communism eventually failed as they were unable to adapt to changing global scenarios. (Cuba, China, and Vietnam have become flexible for survival.)

The ideology which was supposed to set people 'free' started crushing the opposition with an iron hand. (USSR, China, and Cuba)

Communism just became fascism without religious hatred and private ownership.

Conclusion

In my view, Communism is an abstract concept. They dream of a society of equality and justice. The dream is quite beautiful.

But we live in the real world. There is no place of abstraction in day-to-day governance. As with personal life, in governance, we need clarity of thought and realistic goal setting.

By rhetoric of revolution, they come to power with a lot of violence. after coming to power, they face the challenge of governance and policymaking. And governance and policymaking is not an easy thing to do. A certain level of competence and flexibility is required for this.

The incapability leads many to disillusionment with their ideology. So they utilize their power in pursuing pleasure and wealth for themselves.