why are they all men!
This is a good site that explains it:
The question why women can't be ordained priests is often confused with the issue of equality. The Holy Father has made it clear that men and women (as far as their sex is concerned) are equal before God (e.g., Mulieris Dignitatem 6). But equality isn't identity. Men and women have different though complementary functions. Priesthood is a male function, for the reason that a priest is an icon of Christ, and Christ is male. The maleness of Christ is an important sign of His relationship to the Church, His Bride. As in nearly all cultures a man takes the initiative in winning a wife, so Christ took the initiative in winning souls and establishing His Church. For this reason, marriage is a “mystery” or sacrament of the Church (Eph 5:32).