Apple’s product design cycle is pretty long.
The design of the iPhone X was locked down in November 2016... http://mashable.com/2017/10/31/how-apple-built-the-iphone-x/#zDYeeFFzCSq3
“As far as last-minute design changes? Actually, we didn’t have time for it,” said Riccio, who seemed energized by the memory of that intense development period. “Quite frankly, this program was on such a fast track to be offered [and] enabled this year. We had to lock [the design] very, very early. We actually locked the design, to let you know, in November,” said Riccio before he was cut off by Apple PR. Riccio appeared to realize he’d said maybe too much, and then reaffirmed with a smile, “We had to lock it early.”
and it was released in November 2017. So, given the amount of new features in the iPhone X, perhaps HPUE was seen as a stretch.
(This also means that next years’s Devices are likely already locked down.)
Apple needs sufficient component availability to be able to deliver millions of devices at the margins it wants to. So, it purposefully lags behind others while it waits for enough components it wants to be available at the pricing it wants.