Because he's become way too predictable and vanilla in a society that wants people to stand up and talk about politics.
But you're right, he shouldn't delve more into politics. And not because it's "controvesial" (why would Colbert be doing so well if "controversial" was a bad thing?), but because he's a bad host and bad interviewer. He doesn't know how to adapt to day-to-day topics as well as any of the other late night talk show hosts, including Conan.
The reason he got the job was because he got good ratings from his bits and sketches, not because he does a good monologue or is a good interviewer. He should stick to what he's good at, not pivot to what he's weak at.