For one, there is wasted space on the 1280x1440 screen down the dividing line of the headset where the screen is unused entirely. Using dual screens, they can position the screens themselves so that the actual resolvable area of pixels is largely similar between the two.
They're both inadequate.
I tried the DK2 and found the 1080 resolution of that headset to be a screen door of big, chunky and distracting pixels. It looked like I was looking at a world made of Legos.
Any improvement on that, even minor, is likely to swing me in that direction. But even 1440 is probably too low. Of course, increases beyond that start to boost the hardware requirements beyond mass market levels for the present day, so we're still a few years away from anything more becoming viable.
But damn is 1080 ugly on these things.