One reason for the decline is the growing popularity of environmentalism, where most of the brighter communists saw more opportunity than shilling for burned out ideologies.

catholicfundamentalism.com makes the point (sometimes too often) that the Trotskyites mostly became environmentalists, where they work to destroy the funding for the hated Stalinists’ vast public education empire.

If they can’t get tenure churning out bilge, they get into environmentalism, where there’s still enough suckers taking it seriously to fund them.