Same reason movies come out during the week; sales are higher on the weekends because that's when more people have time to look for entertainment. You want to be ready and waiting on Friday and Saturday when people start to shop.
It is sort of built into the process of releasing games that your release day sales determine how well you get placed in the listings going forward, so you want to do every little thing you can do to boost those day one sales in order to boost day two sales and get momentum.
