...If he doesn't want to take calls, that's ok (I'd actually prefer that because most callers are idiots)...
Something I will say that is perhaps slightly analogous to deathofthebambino's position is that there *is* pretty obviously a deliberate effort to keep the show adversarial, loud, and somewhat outrageous, and one of the easiest ways to do that is by letting idiot callers skip to the head of the line. Howard Stern and Rush Limbaugh didn't get where they are by airing thoughtful dissertations from Oxford professors, and for good or ill, rightly or wrongly, there is a widespread perception that the way to build ratings is to have polarized, over-simplified shouting matches.
It's entirely possible that Callahan's schtick is just that, and that by day and off the air, he's a mellow, good-natured, vegetarian pacifist who reads the classics in ancient greek and sings secular anthems of inclusion at the Unitarian Church (or whatever, not sure I would like that guy any better, but you get the idea).
I agree entirely though with the premise that what matters for purposes of the discussion at hand is his on-air personality. He always seems to be in a bad mood, and angry at anyone who isn't. Even when I agree with him, he is exhausting and painful to listen to. His outlook is cynical, grouchy, accusatory, and more than a little abusive. Schtick or no, who likes a guy like that, whatever his opinions? Who wants to hang out with him first thing in the morning? It'd be one thing if he were a particularly witty, insightful, funny, charming, or original curmudgeon, but he's not. He's just a meanspirited prick with an apparent little guy complex. The only answer I can think of is equally unhappy and unpleasant people who find his vitriol validating, or maybe people with a meanspirited or tweaked sense of humor who find shouting matches intrinsically amusing.
His worldview has none of the optimism, none of the faith in his fellow man, none of belief that the best is yet to come that defines the best conservative thought. It sometimes seems as though he is "conservative" because that outlook offers the most opportunities and justifications for punishing people. He appears to have essentially zero grasp of economics except for a generalized hostility towards people who are significantly richer or poorer than the upper middle class, and his knee-jerk dislike of anything and everything civil-rights teeters on the brink of outright racism. At times it seems that any improvement in black or poor neighborhoods or schools is bad news to him (unless it can be directly attributed to republican policy), as though the enemies are taking something from him. Which is, of course, exactly the opposite of free-market economic theory. He simultaneously seems to really have it in for the very rich and the super-successful, again, in stark contrast to conservative economics. For that matter, it generally seems as though anybody who isn't him or just like him must be up to no good and getting more than they deserve. If he were not making any money, I suspect he'd be a communist.
His expressed knowledge of sports is adequate for general conversation at best. If I were a gambling man, I'd bet pretty good odds on JMOH's contention that the majority draw to their show is circumstance, e.g. the car radios still on the station from last night's game, the regional sports-obsession generally, and the default status of the station in local sports radio.
But honestly, it doesn't really matter to me why people listen to WEEI in the mornings, anymore than it matters to me why many southerners scoop up and eat picked pig's feet by the handful out of public jars on gas station counters. Maybe they'd feel the same way about eating lobster, or whatever. But some things are just not pleasurable experiences, and Gerry Callahan is one of them, for me. Especially at 0600.