This makes sense, of course. The Obamas hold particular significance as the first representatives of a racial minority to occupy The White House, and it's understandable that there would be particular pride in the black community.
But as a general matter, First Ladies have never enjoyed the celebrity status that contributes to top performance on the magazine racks -- most Americans are actually not that interested in "politics as lifestyle." They pay attention to politics when it impacts their own lives, but they don't really nuture the same sort of curiosity about politicians (or their wives) that would drive sales with, say, a flamboyant and/or troubled big-time celebrity.
It could be argued that Michelle Obama is being overexposed on the magazine covers. What's more, the fact that she continues to appear on so many of them -- despite a lackluster overall track record for sales -- suggests that she's being placed there in an effort to create public interest in her, rather than as a reflection of public interest that already exists.