The shortest answer would be: it didn’t.
It was not the empire that introduced this change, nor was it so sudden. It happened over a certain period of time, as a result of the political situation at the time.
This is a very tricky question, I’m afraid. Scholars still haven’t found common ground even when it comes to the Fall of the Roman Empire. But, however it happened, Europe had to adapt to the change. Empires were weak, governing was lacking clear infrastructure and was decentralized. A new set of rules had to be devised to protect the land, and warrior nobility had the power of deciding. The result was a new set of customs and relations that later gave way to monarchism.