Why can't women be Catholic priests? This is a very good question and it deserves a good answer.
Here's the short answer: Christ had plenty of female followers. Yet, He did not ordain them - only men. It's Christ's decision and the Church merely follow his pattern.
Moreover, Mary is the greatest human being ever created, and yet she was not a priest. If any human "deserved" to be a priest, it was her and yet she was not a priest.
The reason for a male-only priesthood is that the Church if the "Bride of Christ". Christ is the "Groom". Thus, it is necessary for those who represent Christ (i.e. priests) to be male and represent the Groom to the rest of us who are the Bride of Christ.
Moreover, the physical body of Christ is male and thus in the Eucharist it is appropriate for a male to utter the sacred words: "This is my Body".
I find that the Mary argument works best with Catholics. It shows that priesthood doesn't necessitate holiness or access to God. Some of the greatest saints were never priests (e.g. Saint Joseph, St. Therese).