I'm pretty confident that there has never been a discussion of whether to add it or keep it off. So the most direct answer is - because it wasn't there in 1.0 and nobody has asked for it / asked why until now.
We talked about this at the TC meeting today and agreed that it is too late to consider adding this in DITA 1.3. We've just finished one pass through cleaning up the keyref topics and it's too late to go back and add a new base element with @keyref at this point. Kris is adding this as a suggestion that we will consider in DITA 2.0. Until then, the best way to reuse content in the <category> element is either through conref, conkeyref, or through a reused <keyword> as a child element.
Robert D Anderson
IBM Authoring Tools Development
Chief Architect, DITA Open Toolkit (http://dita-ot.sourceforge.net/)
"Julio Vazquez" ---08/18/2014 06:28:22---While investigating metadata reuse strategies, I discovered that <category> does not have a @keyrefFrom: "Julio Vazquez" <jvazquez@vasont.com>
To: <dita-comment@lists.oasis-open.org>
Date: 08/18/2014 06:28
Subject: [dita-comment] Why does <category> not have @keyref attribute