By having it on the Sundays they got everyone to line up 2 hours before the game so once they get you inside you were buying concessions, jays shop merchandise etc which is a lot of extra revenue even if the game may sell out anyway.
No one's lining up to get in the dome an hour before on a Wednesday in May to get a bobblehead. When they did the giveaways during the week this year you could stroll up at 6:45 and get one pretty easily.