I can't speak for anyone else, but I can tell you why I really like him. First off, after writing my own songs, I really admire him as a songwriter. He does a great job of creating something really catchy, but at the same time it has the appearance of being really simple (that is until you really breakdown what he's doing). It's similar to reggae in many ways. On the surface it appears simple, but when you break down all the parts and pieces, and the overall song's construction, you see the genius of it. It's orchestration with all those parts, yet presenting such an open inviting feeling, the beauty of his songs. Bands can easily put together his music, but most bands are NOT putting these songs together. As a musician it's really easy to sit back and say, 'Pfft, I can play that, and I can make a song like that', but most musicians don't.
Then there are his lyrics. They get in your head. They don't come off as pretentious and heady. They fit their deceptively simple structure. I have a ton of his stuff, and I always smile and enjoy it when I listen, even though this is not the general genre of music that I play.