Why aren't some saints canonized?

The canonization process was not implemented until the 12th century. Before that time any bishop or patriarch could declare a person to be a saint, no approval from Rome was required. In 1170 Pope Alexander III declared that all procedures for naming a person a saint must be examined by Rome and the final decision is that of the pope alone. Eventually the Sacred Congregation for the Causes of Saints was instituted and this office oversees the beatification and canonization process for all candidates for sainthood. They prepare the case and present their findings to the pope who makes the final decision. Saints who were declared such before the process was set up are called 'precongregational saints.' Prior to Rome taking over the responsibility a number of "saints" were proclaimed whose very existence was in doubt and others were made saints who had led lives that were anything but saintly. In 1969 a number of these precongregational saints were removed from the Calendar of Saints. In some cases, local veneration was allowed to continue. However, some had their cults suppressed completely.