The best answer I can give is to say you should read The Road to Serfdom, because Hayek lays out the answer very well and at considerable length.
But I’ll present a hypothetical scenario that covers one of the main points. Imagine we’re in the the Totally Democratic Communist Country (TDCC), and the biggest economic issue is iron ore mining. A democratic election is held and the Pro Iron Party wins, and immediately sets about implementing its iron mining agenda. They seize lands with iron deposits, begin digging enormous mines, recruit and train millions of workers, build the necessary machinery. They are almost ready to begin actually digging up iron ore and then another election is held and the Anti Iron Party wins. The new ruling party is totally opposed to all iron mining, so they shut down the mines, destroy the machinery, fire the workers. Vast amounts of capital have gone entirely to waste.
A real communist state could never allow such things to happen, so what would they do instead? They would step of a Commission of Democratic Mine Administration, which would specifically be insulated against democratic control, and would be allowed to continue with construction no matter what happens in any election. And they would declare that mining iron is a necessary part of democracy and protection of the Glorious Revolution, and therefore that anyone opposed to iron mining must be an Enemy of the People and can therefore be arrested for treason and so forth.
The bottom line is that if you have democracy, then things can always change. The people may choose to elect new leadership and demand new things for any number or rational or irrational reasons. But you can’t have a centrally-planned economy if the central planners and their plans are constantly getting yanked around and sent in contrary directions by democratic processes. Thus communism and democracy are incompatible.
Although the USA today is certainly not communist, one can see the basic problem at work when the government tries big, long-term projects. Some may recall that when President Obama was in office and the Democrats held power in 2009-10, one of their big ideas was building high-speed railroads. The federal government and the state of California eventually agreed on a massively expensive plan to build a new high-speed train route from Los Angeles to San Francisco. Somehow billions of federal and state dollars got spent but no train was ever built, demonstrating the hazards of trying to have the government plan and build something while new elections and democratic give-and-take keep pulling the rug out from under them.