We don't know that Jerusalem was a gubernatorial seat. Even if we accept it becoming a birta' at some point, a birta' is not necessarily the seat of a governor, only of a garrison commander. (Perhaps it is noteworthy that TAD A.7 makes no mention of where in Yehud Bagavahya was but mentions priests in Jerusalem).
We don't know that Jerusalem was a gubernatorial seat. Even if we accept it becoming a birta' at some point, a birta' is not necessarily the seat of a governor, only of a garrison commander. (Perhaps it is noteworthy that TAD A.7 makes no mention of where in Yehud Bagavahya was but mentions priests in Jerusalem).