Why did rome lose its status as the roman empire's most important city as a result of diocletian's reforms?

The reason why rome lose its status as the roman empire's most important city as a result of Diocletian's reforms was Diocletian divided the empire into smaller units, thereby turning Italy into a section of the empire like any other.
This division caused the Rome's area of jurisdiction became significantly smaller and erased its status as the most important city within the empire