Why might ruling an empire with a vastly diverse assortment of people be difficult?

Rulling an empire with a vastly diverse assortment of people can be very difficult because these people have their own beliefs, cultures, principles and needs which make it really hard for a ruler to respond to and keep the empire together. For instance, Charles VI had to deal with this problem since Crotians, Czechs, Italians, Hungarians, and Germans lived within his territory. He spent most of his reign trying to figure out a way to overcome this problem.