I am not sure if this is entirely true, but one thing that I would add is that the iPhones in the past have digitizers that were powered in conjunction with the backlight.
So if you wanted Tap to Wake on the past iPhones, you would have to turn the backlight on all the time; and you don't want that on LCD displays.
On the XR, however, I believe that the backlight and the digitizer is separated, and thus the digitizer can function separately. This also might be the reason why the whole phone is thicker than all the phones in the past few years.