Communism inherently couples both the economy and the government.

In theory, capitalism can be decoupled since it mostly depends on laissez-faire governance. Communism inherently requires a planned economy and centralized control of such.

There is theoretically nothing stopping said leaders of a communist regime from being elected through a democratic process. But much like democracies tend to favor capitalism and (lower case) libertarian ideals, communism tends to lend itself to dictatorships because… you have a centralized control of all aspects of society.