Why poor display quality in all PDF readers except those from Adobe?

Only Adobe and Foxit (of those 5) have subpixel rendering. This is seen by zooming way in on the screenshot and seeing color fringing. This is because each pixel on an LCD display is made of 3 subpixels: red, green, and blue. These can provide extra resolution. Foxit maybe just doesn't have as good of a rendering method. Hinting could play a part.

I think am pretty sure now that it's one of the rendering/smoothing options in Preferences->Page Display. Try changing them (there's quite a few) and seeing if it makes a difference.

Specifically:

If you change the "Smooth Text: For Laptop/LCD Screens" to "For Monitor", you'll see that it'll look like other programs' smoothing.