Diamonds are graded for clarity on a long ladder, from flawless down through a series of lettered grades, because a diamond is prized for being colourless and clear, so tiny inclusions matter a great deal.
A coloured stone is prized for its colour. Applying the diamond ladder to an emerald or a tourmaline invents a precision that does not exist and, worse, makes a stone sound flawed when it is simply a coloured gem behaving like one.
So we grade coloured stones with two words. Eye-clean means no inclusions are visible to the unaided eye. Included means some are. That is the honest distinction, and it is the one that actually matters when the stone is in your hand. Diamonds keep the lettered scale, because for a diamond it means something.