Why was the bishop of Rome typically from the Roman region??? That’s not too hard to answer . . . a more interesting question is why it’s acceptable to pull men from around the world to be the bishop of Rome
Even when popes were invariably Italian, they were not invariably Roman. There were Venetian popes, Milanese popes, Florentine popes, Neapolitan popes, and so on.
As for why it’s acceptable to appoint non-Romans to the see of Rome, it seems to me that the answer to that is because being the diocesan bishop of Rome is clearly not the main function of the pope, even though that is
de jure his primary role. For this reason, the day-to-day administration of the diocese of Rome and the suburbicarian diocese of Ostia is entrusted to the Cardinal Vicar (the remaining six suburbicarian dioceses have diocesan bishops as well as cardinal bishops). Since the pope’s main role is in reality providing leadership for the worldwide Church, it makes sense that popes can be drawn from all over the world. I imagine that the Church and the wider world would probably look very different had a Roman pope been elected at the second conclave of 1978.
Similarly, the Anglican diocese of Canterbury is actually administered by the bishop of Dover. Although the archbishop of Canterbury is nominally head of the diocese of Canterbury, he is principally based in London and is mostly occupied with the affairs of the Church of England nationally and the Anglican Communion internationally, as well as sitting in the House of Lords and serving as a privy counsellor. I have heard it argued that there would be a case for appointing archbishops of Canterbury on a more global basis. Indeed, the current archbishop’s predecessor was appointed from the entirely separate Church in Wales. The counter argument to that is that the archbishop of Canterbury is not supposed to be an Anglican pope and that continuing to appoint English archbishops (and the occasional Welshman) ensures that the role does not take on more power and responsibility than it already has. It also helps to maintain the balance of power between the extremes of liberalism and conservatism within the Anglican Communion. One can only imagine the havoc that would take place if an archbishop were to be appointed from the USA or Nigeria. There is also the practical obstacle that does not apply in the case of the pope, which is that the archbishop of Canterbury is appointed by the prime minister and has to swear an oath of allegiance to the Queen and accept a seat in the House of Lords and a place in the privy council. I suppose you could argue that the Catholic Church would have benefited from having a pope who was more like the archbishop of Canterbury: a
primus inter pares with little real power.