I think Dune was successful because it was a straightforward hero story, whose plot meshed together many interesting sub-themes like politics, religion, ecology

I think Dune was successful because it was a straightforward hero story, whose plot meshed together many interesting sub-themes like politics, religion, ecology, colonialism, drugs, eugenics, AI etc. The sequels failed because they abandoned the simple entertainment that a hero story could provide, and instead put the sub-themes front and center.