I imagine celebrity covers sell better than model covers, simply because the general buying audience is more familiar with celebrities, and are attracted both by the picture and by the coverlines that promise to reveal something about their private life.
Magazines with a smaller circulation, with a more discerning audience, are free to experiment more, as their readers are eager to get to know the newest faces and style, no matter if they're a little extreme.
Everyone at this forum falls into this second category of reader, but we all have friends who give magazines little or no attention, buying them on impulse, flicking through them, and then throwing them in the bin. And that's what most magazines represent to most people - even if they're little pieces of art to us.