Price, performance and battery. A high quality LCD screen with great peak brightness and good contrast, wide color and accuracy is expensive to begin with, then add the antialiased rounded corners, then add the way it doesn't have that one bigger bezel (which, according to OnePlus CEO is doable, but too expensive to make).... so, in order to hit their profit margins, something had to give and Apple chose what they consider the 'least important' spec. I think they made a good choice, rather than compromise in some other aspects.
I suppose that's the main reason - but it's not the
only reason. Battery life and performance aspects are also there, so why take those hits for a minor improvement? It was a smart choice.
"Oh, but why did they do it for X/XS/XS Max then" you ask? Well, they had to - because these are OLED screens that use a pentile matrix that, in practice, needs a higher ppi to look as sharp as an LCD screen. If they could get the same LCD sharpness on an OLED at a lower ppi -
they probably would have the same resolution on the XS, because of battery life (XR has the best battery life of any iPhone).
"Ok, but what about 8 Plus? That's not an OLED?" True - but it's again a case of 'they had to'. The way iOS works is not pixels but points. Not to get too technical, but in order to get everything nice and sharp, you have to have 2x or 3x pixels compared to points. Now, X/XS/XS Max and Plus phones use 3x (the only difference is that Plus phones downsample it, while the X models do not). If they used 2x, at that size, the resolution would be lower than 326ppi - and that
would be noticeable for a phone.
As you see, for many reasons, this was a smart choice. At the end of the day - I'd say it's more important how a screen
looks than how many pixels it has. So it's simple - you walk into a store and see if the XR screen looks good to your eyes. My bet? It looks great, unless you really, really want it not to