Having worked at Target, I've had extensive experience working with their mobile department. Basically, it's run by a third party company called Marketsource (it was Radioshack before, but contract renegotiations fell through and Target dropped them.) This is why their hours differ from the actual store hours, and why even those kiosk hours can vary wildly from location to location.
When I preordered my iPhone 5 from them in 2012, I distinctly remember getting mine in right on time, but a co-worker (who had a different carrier) didn't even get his in. At the time, all logistics and inventory were handled by Radioshack. What changed with Marketsource is that Target now personally handles its own inventory of contract phones (it comes in on our trucks from the DC instead of all phone inventory being sent in through Fedex).
This is where my speculation begins, because I have little knowledge of how Target logistics work outside the store level. My belief is that the Target/Radioshack experiment went so horribly (because of all the inventory issues, specifically surrounding the iPhone 5 launch) that this led Target to seek out a different partner in their mobile business; a relationship that would allow them to control their own inventory to hold themselves accountable for what they have in stock.
I know many here mentioned their horror stories about the iPhone 5S launch through Target. I was scheduled in electronics the day of the 5S launch and everything went off without a hitch. All we had available were pre-ordered units, but we ended up with 2-3 pre-orders that were never picked up (that were held anyway because of the pre-order obligation.)
I've got my 6+ preordered through them, so I'm with everyone here, I hope Target can pull through! Still anxiously awaiting my e-mail. I talked with our mobile manager on Sunday, what she'd been told was that Target has now "sold out" of its initial pre-order shipment, and that any pre-orders from here on out will be for later shipments. She also said what many have said here, that phones "should" be in on the 19th, but they could take up to a week to arrive.