Perhaps because we reaching a point where our eyes can longer perceive the individual pixels that we really have reached maximum resolution. Is there any benefit from higher resolution displays?
The ~235 ppi of the Retina display probably isn't there yet. The human eye is really a nifty organ. For example, it increases the spatial resolution of perception above the density of the rods/cones by sampling from slightly different viewpoints and integrating over time. See: http://clarkvision.com/articles/eye-resolution.html.
Also, there is the fact that you can move your face closer to the screen to look at something small. The eye can resolve over 800 ppi at 4". Finally, the fact that the eye can resolve say 300 ppi at a particular distance doesn't mean that's the best resolution for the underlying screen. If you want to render without tricks like anti-aliasing, you want to have enough resolution so that you avoid visible artifacts. E.g. imagine rendering two adjacent thin diagonal lines separated by 0.3 arc-minutes (the spatial resolution of the eye). You want enough resolution so that you can render those lines without their touching anywhere.
One advantage of high-density displays you sometimes see mentioned is that "we won't need to use anti-aliasing / sub-pixel rendering!"
However at the common "high" density used in cellphones these days, 320+ DPI or so, artifacts in non-AA fonts are still visible (not always, or to everybody, but sometimes anyway).
So, at least, there are some cases where higher-density than today could yield some benefit.
