The right didn’t “let their nutjobs take over,” as Capitalism has continued to decay Capitalists have consolidated power. There wasn’t a cognizant decision to shift towards fascism, but fascism itself arose as the material conditions of society declined.
Fascism doesn’t spread because “it’s an appealing idea,” fascism specifically is a result of Capitalist decline, and pretending it’s just something that happens randomly makes combatting it difficult.