The sacrament of holy orders comprises three degrees: the episcopacy (bishops), the presbyterate (priests) and the diaconate (deacons). The first two degrees — the episcopacy and the presbyterate — participate in the Priesthood of Jesus Christ, and thereby the term sacerdos denotes only bishops and priests. The bishops possess the fullness of the sacrament of holy orders. Priests are their co-workers in the apostolic mission. The diaconate assists and serves them. Nevertheless, all three degrees are conferred by a specific rite of ordination in the sacrament of holy orders (cf. Catechism No. 1554).
In accord with Church tradition since apostolic times, "only a baptized male validly receives sacred ordination" (Code of Canon Law, No. 1024). Since the diaconate is part of the sacrament of holy orders, this regulation applies also to the diaconate.
Rather than stop here with the answer, let’s delve further into "why." In Acts of the Apostles (Chapter 6), we find the apostles in need of assistance. Apparently, some of the "welfare" needs of the church community were being neglected, e.g. the distribution of food among the poor and widows: "The Twelve assembled the community of the disciples and said, ‘It is not right for us to neglect the word of God in order to wait on tables. Look around among your own number, brothers, for seven men acknowledged to be deeply spiritual and prudent, and we shall appoint them to this task. This will permit us to concentrate on prayer and the ministry of the Word.’" Guided by the Holy Spirit, they selected seven men, including St. Stephen, the first martyr of the Church. The Apostles approved these candidates, prayed over them and imposed hands upon them: they ordained these first deacons. Since that beginning of the diaconate, only men have served as ordained deacons.
However, at first glance, St. Paul seems to confuse the matter. In his Letter to the Romans (16:1), he wrote, "I commend to you our sister, Phoebe, who is a deaconess of the church of Cenchreae." In the original Greek text, Phoebe is identified as a diaconos (meaning "deacon"); however, when St. Jerome translated the New Testament into Latin, producing the Vulgate Bible, he rendered the verse, "Commendo autem vobis Phoeben sororem nostram, quae est in ministerio Ecclesiae, quae est in Cenchris," i.e. "I commend to you Phoebe, our sister, who is in the ministry of the Church, which is in Cenchris." St. Jerome refrained from identifying Phoebe as a deacon and instead simply noted that she served in the ministry of the Church. Such a distinction causes one to explore what that ministry of a "deaconess" was.
The best and most information about deaconesses is found in the Apostolic Constitutions also known as Constitutions of the Holy Apostles. Consisting of eight books, the Apostolic Constitutions is the largest extant collection of liturgical and disciplinary regulations from the early Church. Most scholars agree that the first seven books were written prior to 300AD. The eighth was written about 400AD.
From these writings, the role of the deaconess, like that of the deacon, was to care for the Church’s welfare needs. Their role also involved keeping the necessary propriety between the male clergy and female members of the Church. The bishop is exhorted, "Ordain also a deaconess who is faithful and holy, for the ministrations towards women. For sometimes he cannot send a deacon, who is a man, to the women, on account of unbelievers. You shall therefore send a woman, a deaconess, on account of the imaginations of the bad" (Book II, Section XV). For instance, the Apostolic Constitutions assert the following rules: "Let also the deaconess...not do or say anything without the deacon.... So let not any woman address herself to the deacon or bishop with the deaconess" (Book II, Section XXVI).
This point is particularly highlighted in the administration of the sacrament of baptism. In the early Church, after a person professed his faith, the person was then stripped of his clothing and anointed over his whole body with sacred oil. He was then baptized through the invocation of the Holy Trinity and by the pouring of water or being immersed in water. To keep propriety, the bishop would anoint the forehead of those to be baptized, and then the deaconess would complete the anointing of the women. The deaconess was also involved in the immersion of the women to be baptized: the bishop (or priest) was to "dip them in the water" and "...a deacon receive the man, and a deaconess the woman, that so the conferring of this inviolable seal may take place with a becoming decency" (Book III, Sections XV, XVI).
Besides the ministerial role differing between the deacon and deaconess, so does the "ordination rite" recorded in the Apostolic Constitutions. Referring to the actual rite of "ordination" for a deaconess, the following prayer and gesture were prescribed: "Concerning a deaconess...: ‘Bishop, you shall lay your hands upon her in the presence of the presbytery, and of the deacons and the deaconesses, and shall say: "O Eternal God, the Father of our Lord Jesus Christ, the Creator of man and of woman, who did replenish with the Spirit Miriam, and Deborah, and Anna, and Huldah; who did not disdain that your only begotten Son should be born of a woman; who also in the tabernacle of the testimony, and in the temple, did ordain women to be keepers of your holy gates, — do you now also look down upon this your servant, who is to be ordained to the office of deaconess, and grant her your Holy Spirit, and cleanse her from all filthiness of flesh and spirit, that she may worthily discharge the work which is committed to her to your glory, and the praise of your Christ, with whom glory and adoration be to you and the Holy Spirit for ever. Amen"’" (Book VIII, Section XIX, XX). This prayer is substantially different from the prayer for the ordination of a deacon, which is found immediately preceding: For a deacon, the reference to the outpouring of the Holy Spirit is much stronger: "replenish him with your Holy Spirit, and with power, as you did replenish Stephen, who was your martyr, and follower of the sufferings of your Christ." Second, for the deaconess, no mention is made of St. Stephen per se, nor any reference to the apostolic institution of the diaconate. Third, for the deacon, the bishop prays that the candidate may be "worthy to discharge acceptably the ministration of a deacon, steadily, unblameably, and without reproof, that thereby he may attain an higher degree," an indication that the deacon may advance to the office of priest or bishop (Book VIII, Section XVIII).
Therefore, the office of deaconess served a particular ministry to the needs of women in the Church. However, the office of deaconess was never part of the sacrament of holy orders and was not part of the Church’s apostolic foundation. For these reasons, only men may be candidates for the diaconate.