Your subject asks why, so I'm going to answer somewhat literally. May not be the answer you were looking for.

Likelihood #1: people saw the reason, saw who alleged it, and accepted it on face value given the simplicity of the original node, and just voted to reap without seeing the dupe. After all, if you've seen one of a dupe, you've seen 'em all.

Likelihood #2: people saw the reason, tried to go to the other node, but when the link was malformed, gave up and ignored the node (giving more weight to the first group).

Likelihood #3: people saw the reason, tried to go to the other node, but when the link was malformed, typed the number into the search bar, got there, and were confused as to what was going on. This left them in a state of shock where they just didn't go back to the moderated node to vote "keep".

In reality, I think it was mostly #1 - because that's easy and fast, it probably didn't take very long for the node in question to be reaped.

Personally, I would have voted merely to ensure it wasn't approved, if such a vote were available. The node has little value, especially when combined with the earlier node it was alleged to be a dupe of, as the petitioner did not seem to follow the advice of the earlier node (which makes it somewhat of a dupe: ask a different question, but still get the same answer: "How (not) to ask a question" - but that's a dupe of the answer, not the question, which is what How (not) to ask a question was written in the first place, I think). And maybe others had a similar desire, which gave them too much apathy to vote to keep.