Why have nation-states been more successful, as political units, in modern world history then multinational empires have been?

Empires had a meaning when land was the prime wealth of a state. The more the land, the more the peoples, the more profitable the taxation. As of the Renaissance however, the bourgeoisie had begun to prove that, better than land possession, trade and exchanges were the key to a wealthy nation. Because multinational empires had become antihistorical.

It taught people how to live a good life, but not what to believe.