Why was Michael I Rangabe (811-813) the first emperor to have a last name and why didn't any of the following emperors have one until Romanos I Lekapenos (920-944)

Was there any particular reasons last names started to be used more in the 9th and 10th centuries and later on became the norm and not the exception I guess is also my question.

Edit: I know most of early Roman emperor's had last names and other names. I'm more specifically talking about them not being used by the time of the byzantine period and wanting to know why they came back after being gone for so long.