What is the history and explanation as to why the "diaconate" was MIA in the Western Church over a certain period of time?

It was not deacons, but "permanent deacons," who disappeared for a while. In this answer I will summarize and quote from an article titled Called to Serve: The Mission of the Permanent Diaconate by Deacon Michael Chesley.

Although the permanent deacon is a relatively new occurrence experienced on the altars of many U.S. dioceses around the country, deacons in general are nothing new to the Church.

Chesley summarizes the early development and documentation of the role of deacons. As the catechism says (1570), "Among other tasks, it is the task of deacons to assist the bishop and priests in the celebration of the divine mysteries, above all the Eucharist, in the distribution of Holy Communion, in assisting at and blessing marriages, in the proclamation of the Gospel and preaching, in presiding over funerals, and in dedicating themselves to the various ministries of charity."

However, by the time of Pope Gregory the Great in 595 A.D, many of these duties already began to be heavily curtailed or delegated to the minor orders. Although it is beyond the range of this article to cover a detailed account of the reasons for the decline of the order, suffice to say that their [sic] was a gradual occurrence of men who preferred not to remain a deacon all their lives, and wanted to advance to the higher orders. By the middle ages, the order of deacon as a permanent rank in the hierarchy of the Church all but disappeared in the West, and became nothing more than a stepping stone for preparation to the priesthood. Although not widely known, the Council of Trent (1545 – 1563) called for the restoration of the permanent deacon. Unfortunately the idea was never followed through, and it was not until the Second Vatican Council that the Church in the West took up the wishes of Trent some 400 years later.

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Although the Latin Rite Church had "transitional" deacons, the Council goes on to say that the Church envisions what it hoped would be a permanent rank reestablished within the Latin Church. One could ask why did the Church desire to restore the order of deacon as a permanent rank at this point in church history? The Directory for the Ministry and Life of Permanent Deacons addresses this question as follows: "One of the fruits of the Second Vatican Council was the desire to restore the diaconate as a proper and stable rank of the hierarchy". On the basis of the "historical circumstances and pastoral purposes noted by the Council Fathers, the Holy Spirit, protagonist of the Church's life, worked mysteriously to bring about a new and more complete actualization of the hierarchy which traditionally consists of bishops, priests and deacons. In this manner the Christian community was revitalized, configured more closely to that of the Apostles which, under the influence of the Paraclete, flourished as the Acts of the Apostles testifies."*

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Although for many centuries the deacon ascended to higher orders after ordination to the diaconate, there still exist "transitional" deacons and "permanent" deacons. However, there is no sacramental difference or function between the two. Under normal circumstances, when the permanent deacon path is chosen, the deacon remains in his chosen vocation.

Also at Vatican II, married men were permitted to become deacons. Unmarried men becoming deacons are still required to take a vow of celibacy, however.

_{* Chesley's source for this quote is given as: Congregation for Catholic Education; Congregation of the Clergy; Directory for the Ministry and Life of Permanent Deacons. ( Libreria Editrice Vaticana 1998), Chapter III.}