He has the most popular program only because he has the best time slot.
The time slot helps, but BOR has the most popular program in large part because he has absolutely zero competition. If you're a conservative or any other thinking person and you cannot bring yourself to watch the pro-democRAT propaganda that passes for programming on all the liberal networks, what other choice do you have but to watch BOR?
If and when a network decides to air programming during BOR's time slot that isn't far left, programming that conservatives would want to see, O'Reilly's ratings would drop like a rock.