Used to listen to him all the time starting the summer of 1990.
I think three things took the wind out of his sails: (1) the utter duplicity of the GOP after the 1994 congressional victories (during the past 20 years they've made it clear they're part of the Uniparty); (2) his marriage to Marta and related divorce; and (3) the prosecutorial witchhunt over his misuse of prescription drugs.
He's smart enough to know the tipping point has been reached.
I would add one more:
(4) three years after his misuse of OxyContin, Limbaugh was detained at the Palm Beach International Airport, arriving from a "private vacation in the Dominican Republic" carrying a bottle of Viagra prescribed under a different name. Was Limbaugh's trip actually a "sex tour"?