So i presume that Wembley Park is geared up to handle large crowds and also has the frequency of service.Because Alexandre Palace is too near to the event. It's the extended walking distance that presents stations with a steadier stream of passengers that the crush that would result if the nearest staton was open. See my post #54 above. TfL here been managing large event flows this way for years, generally improving the situation for event attendees, other members of the public travelling and staff.