Theres a few different reasons.
1. At the end of his tenure as an asisstant for the men's program, recruiting went in the tank and he was in charge of that. When he took over for Kemp recruiting went down hill. Had it not been for a legacy like Heatley, the program might have fallen off the map all together.
2. He's been in the womens game for a long time and despite having contacts in the men's game it's not that easy to just jump in and start recruiting again. Established coaching options are already on the men's side right now who have been recruiting for the past however many years.
3. Wisconsin has a pleathora of options available. In college alone head coaches Mark Osiecki from Ohio State and George Gwozdecky from Denver would be prime candidates to return to their alma matters. GG didn't want the job when it last became available but since then he has been flirting with a lot of jobs, including Penn State. You also could maybe snag a guy like Tony Granato away from the Penguins, or John Hynes, or Don Granato, all who have strong ties to the program.
That's not to mention other elite coaching options from around the country who have pretty good jobs but might be looking to go to a huge Big 10 power like Enrico Blasi from Miami, or Jeff Jackson at Notre Dame.
I love what Mark Johnson has done for the womens program, but I'm just not sold that he's the guy if Eaves were to ever leave.