When searching for advice on a more general topic is asking for best practices for gming to broad. Examples like Combat encounters with lots of participants, Social combat, Murder mystery investigations, or even a specific system. There is no specific scenario the asker would be trying to deal with but they would be looking for detailed practical advice on a small subset of possible scenarios.
Yep, that's too broad. Without a specific problem or goal at hand, answers will be all over the place, trying to guess what best practices list is the "right" answer.
The voting system just doesn't work under those conditions, and the Q&A devolves into a poor approximation of a discussion forum. At that point it would have been better served to ask the question on an RPG discussion forum (such as one of these ones) in the first place.
Questions that work well here are those that can have specific answers that can be recognized as having a "best" answer. How can I speed up combat with a large number of enemies? is an example of a question that works well here because it's more specific than the hypothetical “What are GM best practices for combat encounters with lots of participants?” It has a specific problem to solve that answers can target with laser-like focus in their attempt to answer it better than any other answer.
In general, Stack Exchange focuses on excelling at what discussion forums do poorly: having an easily-summarised question that is about a specific problem shared by many, and providing its answer/solution clearly and prominently. Meanwhile, SE has sacrificed many things that aren't necessary for that goal in order to pursue it more effectively, and is therefore bad (sometimes really bad) at what discussion forums are still great at doing. SE aims to complement traditional discussion forums rather than replace them, and we recognise that this means some questions will fit the SE model well and some won't.
A “best practices” question falls squarely into the “best for discussion forums” category, by that measure. More particularly, because it's unclear to a hopeful reader whether reading it will solve whatever particular problem they have, and even if it does have the answer to their problem somewhere on the page — since the page isn't focused on a particular problem — who knows where in the page it will be found. That kind of searchability and focus isn't improved by putting such a question on a discussion forum, but the result of putting it on a discussion forum is still better because the discussion won't be artificially constrained by our rigid Q&A format and ban on discussing the topic or tangents.
Overall, a “best practices” question will rarely be narrow enough or objective (or Good Subjective) enough to fit RPG.se, but that's OK because it would be stifled here anyway. Leaving such questions out of our format also makes more room for breaking such things up into many specific questions, that can then by found by focused searches or browsing the related tags.