When else do you want to play the game? CFL TV numbers are best when the games are played one at a time. If games are played at the same time then the broadcasts have to cut up regionally and numbers drop. You can't watch 2 games at once.
The NFL doesn't have to worry about the gate. The CFL does, so the teams have their preferred start days/times. Sometimes we have Thursday night games and when we do, everyone bitches. So if you have only Friday night and Saturday to play all four games, you will have conflicts and stadium availability issues. You also have Saturday night issues with HNiC from October on.
Montreal in particular has always preferred Sunday afternoon games, especially in the fall. It works for them. Their attendance is strong and their TV numbers are strong too.