Why would Charlemagne not allow his daughters to marry?
According to some historians, Charlemagne had as many as twenty children during his lifetime. Although he appeared to be a liberally progressive father (at least for Carolingian tastes), Charlemagne never allowed his daughters to marry. He feared that competing heirs to his throne would distort his intended legacy and rule.
Meanwhile, Charlemagne's daughters had access to important scholars and were exposed to the court culture in their father's kingdom. Bertha, Rotrud, and Gisela were rumored to have engaged in learned discourse with the scholar Alcuin.
It was said that Charlemagne adored his daughters; their beauty, vivacity, and intelligence captured the imaginations of his male courtiers. Indeed, while Charlemagne did not permit his daughters to marry, he indulged their extra-marital affairs with great affection and lavished grand attention on his bastard grandchildren. Charlemagne's protectiveness even caused one poor king to regret ever tangling with Charlemagne in marriage matters. It was said that Offa, the king of Mercia, was so happy for his daughter to marry Charlemagne's son, Charles, that he made the mistake of suggesting that Charlemagne's daughter, Berta, marry his son as well. The result of this indiscreet suggestion caused Charlemagne to go into a great rage: he called off his son's marriage to Offa's daughter and subsequently prohibited any English merchants in Francia.
Indeed, Charlemagne's son, Louis the Pious, protected the Carolingian line to the throne when he came into power. He sent his sisters straight to the monasteries. Louis wasn't even going to take a chance that any of his sisters might marry and possibly threaten his right to the throne. However, he made a fatal mistake which Charlemagne did not: he allowed his daughter, Gisela, to marry, thereby opening himself up to potential succession difficulties Charlemagne had largely avoided by forbidding his daughters to marry.
The two pertinent links above are taken from excerpts of the books:
Hope this helps!