Because the Cato Institute is calling the CBO "ideologically biased".
I have personally known three CBO directors (Bob Reischauer, Rudy Penner, and the late Ned Gramlich) and met three more. I've also known several CBO employees.
I have encountered several Cato Institute staff too. And I read stuff from both of them. And I like a lot of what the Cato Institute puts out.
But this doesn't even merit being called "the pot calling the kettle black". This is more like "the pot calling the bucket of white paint black".