Newb question but why wouldn't AC use multiple gates for boarding? Ie. Business class boards at gate x, which connects to the fore boarding door. Prem economy and economy board at gate x(+/-)1 which connects to a middle boarding door. For example on the 787 (which is now the most common wide body) business would board on the fore boarding door (L1?) while everyone else boards at the second boarding door just before the premium economy cabin (L2?). Is this a restriction with the airport or the airline?