Under that logic, the X would have come out right away last year, or perhaps earlier than the 8/8+.
Most of the analysts believe the XR was delayed due to component supply issues, and in fact said so several months before the actual launch. The specific issue was the LCD. Not sure what the problem was, but this is Apple's biggest phone LCD, as well as its first LCD with a notch, and also Apple's first display of any sort using the CGS process.
Certainly is a reasonable answer. It was also reported there were software issues which is even more reasonable. But Apple could have just as easily prioritized the Xr with the Xs/Max following a month later. After all, last year the 8/8Plus predated the X...
https://www.wsj.com/articles/apple-leads-with-its-pricier-iphones-this-year-1537349401