If you preorder through Amazon, even if you use Prime 2 day shipping, you may not get the game until Wednesday, August 7th. That's the alert that came up when I added it to my cart early in the week. So Amazon is apparently unable to guarantee release date delivery on this title. I actually couldn't find the release date delivery claim on this product page, which is a first that I've seen for preorders.
I think Nintendo wants you to buy this title on the eShop Saturday at or really close to midnight, which would make some sense given the Sunday release. Nintendo's Japan eShop version has it available since early July and on the US eShop under important information it says "it may take longer to download the digital version of this title..." even though the purchase button isn't enabled yet.
I'm not going to risk it on this one though and I don't want to download this game, I want the box and disc, etc. Target or Best Buy will have it on the shelves Sunday morning when the stores open. The idea that makes sense is what Stephanie brought up, that UPS/FedEx/Ontrac/USPS will not deliver on Sunday, and Amazon can't give it to you the day before the official release date. They won't ship any preorders on Friday because it's too soon. They can't ship it on Saturday either because people like me who live in California will get the game early on Saturday, even with 2 day shipping.
As far as I can tell, for this purchase there is no benefit from getting the game from any specific retailer, such as bonus DLC or anything like that, except that you might save on sales tax but not in my state. That and the last time I preordered a game it was sent from Amazon to me by Ontrac. I have cameras and was home but they said they attempted delivery. I received that game 2 days later than all my friends. Might as well not let it be an issue and just get it at a store. When you preorder at a game retail store, it's like being a VIP because they'll hold a copy for you. You can just waltz up to the counter and give them your info and walk out with the game before the late comers. It's pretty sweet. But if it's too late to preorder in your local stores, I would just go into a less obvious retailer early Sunday morning, like Kmart or ask a friend with a Sam's club or Costco card to help you get a copy of the game.