Education and learning was largely preserved during the Middle Ages through the Church and through the progress made during the age of the Roman Empire. The language of academia, for both of these reasons, was in Latin.
People who went into academia, learning to read and write and study other subjects, usually did so in Latin. This is both because the Roman Catholic Church usually administrated this education, and also because much of the texts that were studied were in Latin due to there origin in the Roman Empire.