Complex sentence: The closeness is at least in part because of the influence of Cusco Quechua on the Ecuadorean varieties in the Inca Empire, as northern nobles were required to educate their children in Cusco, which maintained Cusco as the prestige dialect in the north.
Split into simpler sentences: The closeness is at least in part because of the influence of Cusco Quechua on the Ecuadorean varieties in the Inca Empire. Because Northern nobles were required to educate their children in Cusco, this was maintained as the prestige dialect in the north.