The music is what keeps them coming back, of course, and much has been said about its appeal. If Steely Dan is not quite like any other band, its hold on fans is not quite like any other band's, either. Their influence is so great, in fact, that fans turn easily into loyalists, defending the standard against all critics and keeping the band in heavy rotation even when newer disks crowd the shelf. A self-consciously intelligent, goony kind of fan base. I think they attract other types, too, but also genius lunatics.
Objectivity demands a couple of caveats.
First, this is an anecdotal survey, not a comprehensive one. Steely Dan has sold millions of albums, and there are probably at least four or five buyers out there who didn't want anything deeper than, say, background music for getting down. Then there's uniqueness. To pick just a couple of examples from popular music, Frank Zappa and Randy Newman have cultivated notoriously loopy and intellectual followings. Becker and Fagen, first and foremost, are populists, making sleek, funky music anybody can groove to.