Because pre-orders placed earlier today are already pushed to the end of November (hopefully it won't actually take that long).
Because there were numerous reports of the 6+ production being delayed quite a bit.
Because I have a feeling this is the gold iPhone all over again.
If you go to an Apple Store there may be some. Anywhere else, I say no chance. I'm so sure of it I'm not going next Friday and I'll just hope my pre-order gets bumped up from its current mid-October shipment.
Obviously no one knows for sure. If I'm wrong and you pick one up next Friday you can come back and gloat about how much you love it and how wrong I was.