As Pierre Riché points out, the expression "Carolingian Renaissance" does not imply that Western Europe was barbaric or obscurantist before the Carolingian era. The centuries following the collapse of the Roman Empire in the West did not see an abrupt disappearance of the ancient schools, from which emerged Martianus Capella, Cassiodorus and Boethius, essential icons of the Roman cultural heritage in the Middle Ages, thanks to which the disciplines of liberal arts were preserved. The 7th century saw the "Isidorian Renaissance" in the Visigothic Kingdom of Hispania in which sciences flourished and the integration of Christian and pre-Christian thought occurred, while the spread of Irish monastic schools (scriptoria) over Europe laid the groundwork for the Carolingian Renaissance.
There were numerous factors in this cultural expansion, the most obvious of which was that Charlemagne's uniting of most of Western Europe brought about peace and stability, which set the stage for prosperity. This period marked an economic revival in Western Europe, following the collapse of the Western Roman Empire. Local economies in the West had degenerated into largely subsistence agriculture.
Apart from the stability engendered by the unification of Europe, one of the major causes of the sudden economic growth was the slave trade. Following the rise of the Arab empires, the Arab elites created a major demand for slaves with European slaves particularly prized. As a result of wars and raids in the Eastern Europe, a steady supply of captured Slavs reached Frankish/Italian merchants which were then sold to Muslim Spain and other parts of the Arab world. The trade was so lucrative that it almost immediately transformed the European economies. The market for slaves enabled the West to re-engage with the more civilized cultures of the East (the Muslim and Eastern Roman empires) so that other industries, such as textiles, were able to grow in Europe as well.