Apart from the obvious problems (more pixels = more GPU/CPU power needed), the more dense the screen, the less effective is the backlight: With a denser matrix,

Apart from the obvious problems (more pixels = more GPU/CPU power needed), the more dense the screen, the less effective is the backlight: With a denser matrix, less light passes through the tinier cells, and you have to crank up raw backlight strength to compensate. And that is absolutely murderous for an outdoor phone that has to have a backlight as bright as possible, so you can read it in the sun.