Bloomberg Businessweek editor Josh Tyrangiel says in an emailed statement that they chose not to use the cover for editorial reasons:

Each week we design a few cover possibilities for the upcoming issue. We see where the news is going and make a final decision about the image on Wednesday afternoon. Two weeks ago (January 16 issue, on stands January 13) we had two great pieces in the magazine: Ashlee Vance's profile of Steve Ballmer and his efforts to reboot Microsoft, and Peter Coy's Opening Remarks about how the GOP had turned on Mitt Romney and Private Equity. In the end,we went with Ballmer since the Romney story seemed to have already hit its peak. A lot of times these decisions are about all of us putting our finger in the air and trying to figure out which way the zeitgeist is blowing. It's what makes the job so fun and so challenging.