Because we've had 5nor 6 of these standards and it becomes unclear which one should take precedence. If FORCE_COLOR=1 and NO_COLOR is defined, which one wins?
FORCE_COLOR was the first we had found that was also present in other Unix utilities and thus the one that won.
I'm a bit late, but perhaps you could write the no-color website maintainer, and ask to link to similar projects? Then we could see all options to disable or forcibly enable color in one glance.
FORCE_COLOR was the first we had found that was also present in other Unix utilities and thus the one that won.