It is logical, as I'll now explain.Literally every big game the whole season is either twilight or at night except the grand final, it isn’t a logical thing to say the GF must be retained during the day.
During the year, there are a lot of games of footy, and there are a lot of things for us to do, including playing our own sports among other things.
We work our footy watching in with our schedule, and having the big games at night enables more people to watch them.
During grand final week, and then of course grand final day - it is the other way around. It's all about the footy, the grand final IS what we do, and we work other things around that. The grand final has the power to choose the best time. And the best time, for reasons many have said here already, is the afternoon.