Let me take a crack at answering this, being the moderator in question. There are essentially two reasons:
The question is subpar. The fact that it was later closed, reopened, and attracted more closing votes now show that I am not the only person who felt this way. I don't think it's necessarily a bad question, that's irrelevant. But I don't think this is the sort of question we want to advertise on the HNQ list as representative of our site.
Arguably, the title should contain $\rm\LaTeX$ anyway.
You can learn more at the link Martin posted in the comments to your question here, where I explained my view on removing HNQ status.
Moderators frequently remove questions from the HNQ. Our site is large and provides many interesting questions. The number of questions per site is limited. Removals increase the turnover of math-questions in the HNQ and thus allow more math-questions to get known network-wide.
Put differently it seems better to have eight questions there for three hours each, rather than one for a full day. Except maybe if that one question is stellar, and thus likely to be much better than those that will replace it; with all due respect that does not apply to the question you mention.