A historical question: What reasons might there be for a clan NOT having their own tartan? They annoyed the tartan-maker? Didn't get the memo? They decided it was just a fad?

By marriage, I'm a Douglas, but by birth, I'm a Creighton/ Crichton (the line that transplanted to Pennsylvania, though I grew up in the town where the Illinois line transplanted to). As far as I can tell, Crichton has a clan crest, but no tartan to call its own. They wear the Edinburgh. I'm curious to know why this might be. Any thoughts?