I also believe that Apple overestimated people that were going to be buying Iphone 8 and still have manufacturing capacity consumed by making Iphone 8 when there is little demand for those. An article I read said that Foxconn/Hon Hai would be cutting production of Iphone 8 to make room for the 10 but that wouldn't happen until Q1 2018.
makes sense, i guess they thought only enthusiast would go with the iPhone X.