it's also a matter of size. lores graphics are 1k (they usually compress at least 50%). There are 800 backgrounds in the demake. You want to fit this on a reasonable number of floppy disks, in this case it fits on 3.
If you use "hi-res" (140x192x6 colors) the graphics are 8k, 8 times bigger, so you need 24 disks which is a bit much.
Also with double-buffered hi-res graphics you waste 16k of RAM which is 1/3 of all available RAM on a II+. Double-buffered lo-res only takes 2k.