Communism will always be attractive to someone of average or below-average ability. Who would turn down free stuff?

Libertarianism will never be attractive to more than the top couple percent, so can never win elections.

Our economy is so efficient that one person with a real job can support multiple welfare freeloaders. As long as that is true, and it's a democracy, communism will be powerful.

I.e., person with a job votes to keep his stuff. People freeloading off welfare vote to take from the person who works. There are more freeloaders than people with a job, so the workers are stuck.