In the early days most DJ's who remixed her stuff were men, I don't think there were many women in the game back then, on top of the fact that I remember Madonna herself saying she didn't care much for remixes, and I think it may have been more WB commissioning the remixes and Madonna probably approving it. I think Shep may have been the first guy she asked to do her stuff, though not even sure she was the one that brought him on for LAP, the way they both talk makes it sound like it wasn't Madonna who brought him on board, but she liked his stuff and then worked with him.
In recent years I'm not too sure why. I guess we can't be sure she or her team haven't approached certain women and maybe they have turned her down or there have not been any women who Madonna was particularly interested in for her music.