That premise is loaded with error because there arent that many libertarians to begin with but among them there are indeed some women..ergo the question should be
"why are there not as many female libertarians"
I postulate the following..first libertarian-ism takes alot of abstract thought,imagination and analysis..fields women typically dont excel in..Women just cant or dont usually like to think that deeply about abstract subjects..can the average person recall any other prominent female philosopher other than Ayn rand ?
Second libertarian ism is ideologically opposed to modern feminism since modern feminism is about diverting the power of the state to serve womens objectives, Things like alimony,child support redistribution and state funded womens shelters and programs are not libertarian goals
Third if we accept that the female mind works differently then its easy to see females tend to be more pragmatic and take far fewer risks than men ...
Millions of years of evolution have conditioned men and women's minds to work differently..this can easily be seen by how dangerous careers tend to be male dominated
The thing about unlimited freedom is that its heights are limitless, and its lows are bottomless.Libertarians must accept that risk that they can succeed spectacularly or end up a nameless skeleton at the bottom of a ditch.
Women look at that hobbesian world where the strong survive and the weak may freeze or starve to death and are horrified at the possibilities so they tend to prefer the relative safety of collectivist systems despite the trade offs.