You can find the explanation here, in Italian. In this case, we will wait for the official translation by the Vatican Information Service, to be released in a few hours. The main point is that the term was "obsolete" and did not properly reflect the structure of the Latin Church or the current meaning of "West".
Update - Vatican report:
Update - Vatican report:
VATICAN CITY, MAR 22, 2006 (VIS) - In the wake of media comments concerning one of the Pope's titles - that of "Patriarch of the West" - which did not appear among the list of papal titles at the beginning of this year's edition of the "Annuario Pontificio" (pontifical yearbook), the Pontifical Council for Promoting Christian Unity has issued a communique clarifying the reasons for the omission.
"From a historical perspective," the communique reads, "the ancient Patriarchates of the East, defined by the Councils of Constantinople (381) and of Chalcedon (451), covered a fairly clearly demarcated territory. At the same time, the territory of the see of the Bishop of Rome remained somewhat vague. In the East, under the ecclesiastical imperial system of Justinian (527-565), alongside the four Eastern Patriarchates (Constantinople, Alexandria, Antioch and Jerusalem), the Pope was included as the Patriarch of the West. Rome, on the other hand, favored the idea of the three Petrine episcopal sees: Rome, Alexandria and Antioch. Without using the title 'Patriarch of the West,' the Fourth Council of Constantinople (869-870), the Fourth Lateran Council (1215) and the Council of Florence (1439), listed the Pope as the first of the then five Patriarchs.
"The title 'Patriarch of the West' was adopted in the year 642 by Pope Theodore. Thereafter it appeared only occasionally and did not have a clear meaning. It flourished in the sixteenth and seventeenth centuries, in the context of a general increase in the Pope's titles, and appeared for the first time in the 'Annuario Pontificio' in 1863."
The term 'West' currently refers to a cultural context not limited only to Western Europe but including North America, Australia and New Zealand, thus differentiating itself from other cultural contexts, says the communique. "If we wished to give the term 'West' a meaning applicable to ecclesiastical juridical language, it could be understood only in reference to the Latin Church." In this way, the title "Patriarch of the West," would describe the Bishop of Rome's special relationship with the Latin Church, and his special jurisdiction over her.
"The title 'Patriarch of the West,' never very clear, over history has become obsolete and practically unusable. It seems pointless, then, to insist on maintaining it. Even more so now that the Catholic Church, with Vatican Council II, has found, in the form of episcopal conferences and their international meetings, the canonical structure best suited to the needs of the Latin Church today."
The communique concludes: "Abandoning the title of 'Patriarch of the West' clearly does not alter in any way the recognition of the ancient patriarchal Churches, so solemnly declared by Vatican Council II. ... The renouncement of this title aims to express a historical and theological reality, and at the same time, ... could prove useful to ecumenical dialogue."