Some ideas.
People didn't/don't like Rumsfeld.
In the quote's original context, Rumsfeld used it as the basis of a non-answer to a question:
In regard to Iraq weapons of mass destruction and terrorists, is there any evidence to indicate that Iraq has attempted to or is willing to supply terrorists with weapons of mass destruction? Because there are reports that there is no evidence of a direct link between Baghdad and some of these terrorist organizations.
[snip]
Q: Excuse me. But is this an unknown unknown?
Rumsfeld: I'm not --
Q: Because you said several unknowns, and I'm just wondering if this is an unknown unknown.
Rumsfeld: I'm not going to say which it is.
People think Rumsfeld's particular phrasing is funny, and people don't judge it as insightful enough to overcome the initial 'hee hee that sounds funny' reaction.
However insightful the quote is, Rumsfeld arguably failed to translate it into appropriate action (or appropriate non-action), which might have made it seem simply ironic or contrary rather than insightful.
(Edit to fix formatting.)
People think Rumsfeld's particular phrasing is funny,
So what would be the non-funny way to say? IMHO, Rumsfeld's phrasing is what you get if you just say it the most direct way possible.
This is what always bothers me: people who say, "hey, what you said was valid and all, but the way you said it was strange/stupid". Er, so what would be the non-strange/stupid way to say it? "Uh, implementation issue."
Rumsfeld used it as the basis of a non-answer to a question...
In the exchange, it looks like the reporter's followup question is...
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