From NPR.

The network says it was purely a financial decision.
The live audience at New York’s Ed Sullivan Theater was not happy to hear Colbert deliver the news — that this will be his show’s final season. Colbert has hosted the show since 2015, taking over for David Letterman.
The news comes shortly after CBS paid $16 million dollars to settle a lawsuit filed by then-presidential candidate Donald Trump, whom Colbert regularly skewers in his monologues. In a statement, the network said the decision to end the show was “not related in any way to the show’s performance, content or other matters happening at Paramount” — CBS’s parent company.