When this mod came out, minecraft beds existed only in red, so having the curtains only in red too was the natural choice. But since minecraft 1.12 now have beds in all colors, it would make sense for the curtains to also be available in all colors to match the beds.
I usually use blues and green for my decorations and red clashes horribly with those colors, so now I only miss colored curtains to put my favorites colors in the bedrooms too...