Your Question: What is the reason historically and currently that women are not able to be priests?

The Church's teaching that women cannot be ordained priests is, of course, one of most asked questions in the Church. For many, this teaching of the church is synonymous with centuries of mistreatment of women, and second class status in society. It is true that members of the Church, all of us sinners, have in the past and even today have twisted the Bible and teachings of the Church to further agendas that claim women are inferior to men. Our Church must work to heal these wounds, and promote the great dignity and worth of both women and men, and the great love Christ has for all of us. The wounds run very deep, and there is a lot of healing that needs to be done.

The historical and current reasons for why women cannot be ordained, however, can be distinguished from the sinfulness of past generations. It hinges on the understanding that women and men are indeed equal in dignity, but that there are, of course, distinctions bewteen the sexes. Both men and women must come to know what calling Christ gives them in the Church, as men and as women. The history and tradition of the teaching is more complicated that we realize, and often we don't take the time to learn the "why" behind the teaching. So, I appreciate your question very much! I will refer you to the following articles, which treat the subject much more in-depth that I can here.

The follwing article presents a summary of why there is no women's ordination, offering responses to common statements about women's ordination:
http://www.catholic.com/thisrock/1996/9601fea3.asp

The following article disucsses the nature of the sacrament of ordination, and why the Church can't ordain women:
http://www.catholic.com/thisrock/2001/0105fea4.asp