His comic appeal to people who merely believe themselves to be above average.
It's got nothing to do with intelligence, or even knowledge in a general sense. It's that his comics so often rely on specialized knowledge. For example, a couple of my favorite strips are the "sudo" strip and the "Bobby Tables" strip. The former is only understandable to someone who has at least a passing acquaintance with *nix system administration, and the latter requires some knowledge of SQL and SQL injection attacks. Neither of those things is hard to understand. They don't require great intelligence. But they're not generally known. And to people who require an explanation, they're not funny (I have t-shirts of both, and I have never gotten so much as a chuckle from anyone to whom I have to explain the basis for the jokes).
You'll note, of course, that I'm not actually addressing your real point, which is a snarky argument that only people who like to feel themselves smarter or more knowledgeable than most would enjoy the strip. That's because it's not worth addressing.