Writing about what has been called “The First Reformation”, Diarmaid MacCulloch says:
The idea of a Reformation in the sixteenth century is probably a familiar one, but it must be realized that an equally crucial Reformation took place in the Western church during the eleventh and twelfth centuries. It was not a rebellion in the ranks like the [16th century] Reformation; on the contrary, it was directed from the top of the church, and it resulted in the creation of the most magnificent single structure of government which Christianity has ever known. Whether we approve of this achievement or not, it deserves the title of Reformation just as much as the actions of Martin Luther and John Calvin (MacCulloch, “Groundwork of Christian History”, London, UK: Epworth Press, ©1987, pg 105).
The timing is important to note. Consider that the Roman Catholic/Eastern Orthodox split had just occurred with mutual anathemas in 1054. Those in charge at Rome had just cut themselves free from the Eastern Patriarchs who, for centuries, had resisted Roman claims to exclusive power. Now they had a chance to enact their own agenda.
Popes in the past had been appointed in a bewildering variety of ways – elected assemblies of clergy and people, hailed by acclamation at the funerals of their predecessors, nominated by local gang-bosses, appointed by emperors.
A synod at the Lateran in April 1059 promulgated a new papal electoral decree, confining the actual choice to the seven cardinal bishops, with the subsequent assent of the cardinal priests and deacons, and then the acclaim of the people: vague and grudging provision was also made for imperial approval.
The ‘cardinals’ were simply the senior clergy of Rome. The word itself is probably derived from the term for a hinge or joint, and was first given to the twenty-eight parish priests of the titular churches of Rome, who also served the five papal basilicas which collectively formed the Pope’s cathedral. This double role made these priests the ‘hinges’ between the See of Rome, and the parishes of Rome. The word lost this precise original meaning, however, and became an honorific sign of status.
It was extended to the holders of the seven ‘subarbican’ bishoprics round Rome, and to the nineteen deacons of the city, and it was this group of fifty-four senior clergy which was now envisaged as the sole electoral body for the papacy, with the initiative and determining role going to the cardinal bishops.
The decree was a clear attempt to exclude lay influence, whether from the Emperor or the Roman nobility, from the whole process.
The same synod made clerical marriage illegal, ordered the laity to boycott the Masses of priests who kept concubines, forbade the acceptance of churches from lay proprietors, and stipulated that the clergy serving a great church should live in common, a move towards a monastic pattern intended to improve clerical morals and discipline (Eamon Duffy, “Saints and Sinners: A History of the Popes”, New Haven and London: Yale Nota Bene (Yale University Press) ©2001 edition, pg 118).
For more information, see also this article: What are Cardinals?