Deliberate exclusiveness; intentional snob appeal. This shows in the first cover's pretentious and off-putting design; with the footnotes; and in the pseudo-scientific essays constructed to drive away the hardiest ordinary reader, leaving those who'll buy the publication for other reasons. Eggers went after the snob segment of the young literati and he got it.
The Believer has a slightly different mix, where the complex pretentiousness has been toned down a trifle, the narcissism (an essential part of the Eggers phenomenon) bumped up. The cover displays literary posers (drawn in posed, self-loving fashion): the artistic equivalent of David Berman's poems.