Why has Marxism been widely believed?

Marxism as a testable theory has been repeatedly debunked, disproved, and dismissed. But many people still believe it.

A possible reason is that Marx told a simple and compelling story. Like many religious stories, Marx's story gives a purpose to humanity. The story frames a "big picture" view to those confused by the world's randomness.

Marx used Hegel's framework of linear progression to describe steps to a social utopia.

It's not useful to point out Marxism's failures or fallacies if the believers accept it on faith.

How will a new theory be developed that captures the faith of the average person? In a world of seven billion people, a movement of a small minority, say 10%, is still huge.