Just as the clan tartan is an indication of a clansman's status as a member of the clan.
This is somewhat related to a point raised earlier in the thread, but the feathers in one's bonnet is an indicator of an individual's status. Not a group. So the comparison isn't exactly the same.

Furthermore, there was no tradition (that I know of) that involved wearing a feather in one's bonnet before it became a chief thing. With tartan, though, the precedent had already been set that anyone could wear anything, and it was not an indicator of any particular affiliation.

In other words, tartan started out being worn as "general fashion" and was later claimed (by some) as representing some sort of entitlement. Feathers didn't (unless I'm mistaken, and please feel free to correct me if I am).