It was about the ratings and the cost. They could fire him for cause this way and not have to pay out the contract. He's 1/2 million viewers under Colbert, who got canceled for the expense. The rest is window dressing
It was ratings. He's way below Colbert and he got canceled for the cost off the show. This way they don't have to pay out the contract and they make their affiliates happy
They were looking for a reason to broom him and not pay out the contract. He was getting about 1.7 million viewers/night and declining. He was actively driving away viewers as well. For reference, Letterman's worst ratings were around 3.8 million viewers.
