Two things: First, he made plenty of perfectly reasonable points. Any intelligent person can effectively argue both sides of any issue. I often agreed with his base case, before he'd go off the rails with it. And also, as the old saying goes, "the best lies contain a kernel of truth".
But also, remember that he started as a DJ, an entertainer. He brought that DJ sensibility -- which is not brought by people like Hannity, et al -- to his shows, using humor and satire very, very well. So he spoke to a lot of people, and he did it in a VERY engaging way. That's one helluva combination.