Because only men remain in the hunt. Bachmann dropped out long ago due to lack of support.
If you are thinking of Palin, she is primarily a TV personality now. I don’t think she will ever again offer for high national office.
I think that if she is asked, she will serve. IMHO. And, she is the only person that I can think of at this date that can whip Obama. Romney is the only one of the four that can get nominated and that’s dubious unless there are some quitters, but if he does, way too much of the conservative base will sit out the General. Romney can’t win. That’s our only chance that I can see.