The reason, simply put, is that Christ did not give the Church the authority to ordain women priests. It is the Tradition (big T) of the Church that priests have always been men. As with all Tradition, it was passed down by the Apostles, who received it from Christ, though it was not written in Scripture. It is implied, however, in Christ’s choice to only call men to be Apostles.
We can understand this Tradition better when we consider the Sacraments. Each Sacrament in the Church requires valid matter. The Eucharist requires bread and wine. Baptism requires water (normatively). Confirmation requires Holy Chrism. Marriage requires a man and a woman. The sacrament of Holy Orders requires a man.
The immediate question, of course, is, why aren’t women valid matter for this Sacrament? The reason is that Sacraments symbolize what they signify, and signify what they symbolize. In other words, in order to have valid matter, the symbolism must fit the grace received. In the case of Holy Orders, the priest stands as an alter-Christus (another Christ), capable of celebrating the Holy Sacrifice in Christ’s stead.
In order for such a person to be valid matter for the position, they must have attributes that would connect them to Christ. Obviously, Christ was a Man, but it goes further than this. In all of Jesus’ teaching on the Church, He used the imagery of the Bride. He referred to Himself as the Bridegroom. Christ is married to the Church and She to Him. Priests are, in a similar mystical way, married to the Church. It would not be fitting for a woman to be “married” to the Church any more that for her to be “married” to another woman.
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