Their ratings haven't gone down, but somehow...the magic seems to be...well, I can't really say, because I don't know for sure.
Stewart and Colbert are good for different reasons, but their successes were united in one social/political principle: people were fed up with what was being offered by the conservative movement at the time their shows gained traction.
The two hosts shined light, in their own ways, on what people disdained the most.
Their massive popularities and social relevances were born of the Bush years, when a sense of rampant public discontent brewed in more corners of the nation than either political party seemed to expect: the Iraq war, Jack Abramoff, and the assorted ethics scandals of congressional Republicans all crescendoed in 2006 and then 2008, after the Left's disillusion at the crushing defeat of John Kerry in 2004, to lead the voting public to lose faith quite drastically in the governing Republican Party and the ideologies that drove it to power and the country to war in the first place.
Stewart and Colbert were there to point out all those things, Stewart through his dry criticism and penchant for using video footage to display hypocrisy at its most literal, and Colbert through his exaggerated impersonation of Bill O'Reilly and his acute ability to parody all the flag-waving, eagle-screeching, if-you-don't-support-the-war-you're-not-a-patriot, gut-driven commonsense aggressiveness of Fox News.