Today I answered a question regarding why it's impossible to do a 100% kill nightmare speedrun in Doom 2; which to me seemed on topic regarding speedruns, which have been discussed on meta before. Especially because the answer, and the question itself, are really about the inner workings of the game Doom 2, I fail to see why the question was subsequently closed.
My intuition of the on topic rules was that questions regarding mechanics were explicitly allowed, and the details of speedrunning specific games seem to have been accepted as on topic previously when discussed on the meta, if implicitly since the discussions I have found were about tagging them as speedrun.
Why would this question not fall under the limits of these categories?
Alternatively, would relabeling the question as, "What makes it impossible to do a 100% kills speedrun on the Nightmare difficulty?", make the question more directly about the mechanics, and thus on topic? I feel that would make it fit better, but the user asking why a category of speedrun doesn't exist wouldn't be able to phrase it that way without already having the knowledge that it is in fact impossible.