It comes down to Karl Marx's true magnum opus, which was never the
Communist Manifesto but Das Capital:
1. Marx describes a banking system in which periodic collapses such as the one that occurred in 2008 were not only possible, but inevitable. That is certainly attractive, since non-Marxist economists were very much caught by surprise, while the Marxists were not.
2. Marx described a labor system in which the owners of capital would pay workers only barely enough to keep enough workers capable of doing their jobs - in a world without trade unions the vast majority of life experience of Millenials is precisely that.
3. Marx described the capitalists gaining complete control of government. There is serious research, not just a gut instinct, demonstrating that this has indeed happened.
4. Marx described a system in which increasing automation and technology benefits solely the capitalist class, and leaves workers more desperate. This has indeed happened, particularly as a result of the major advances in computing technology since 1980.