I disagree with the notion that history is mostly about "what" and not "why". I think it's the opposite: history mostly deals with "why", supported by evidence from "what". The problem is that history deals with a specific subset of "why"; we need to be careful when dealing with a "why" question, but it doesn't mean we should rule them out immediately.
Imagine a young child asking a series of "why" questions. At first the answers are trivial, but eventually they get so hard as to be unanswerable. Good history questions are like those in the middle; on one end they are trivially answered by Wikipedia, on the other they veer into social science or philosophy. We have close reasons for them already.
Many historical sources and professional historians deal with "why"s directly. For example, Records of the Three Kingdoms contains this appraisal of Guan Yu:
然羽剛而自矜，飛暴而無恩，以短取敗，理數之常也。 That is, "Guan Yu's defeat was due to his arrogance." If someone were to ask "Why was Guan Yu defeated in Fancheng?" then there's a direct answer from a historical source.
What I'm proposing is that we go easier on "why" questions. We're already quite tough on newbies; this is understandable as it's a defense against the disproportionate amount of crackpots and amateurs we attract (guilty myself, but I'm trying to learn!) but please don't knee-jerk react when you see "why" in the question. A lot of these questions attract really poor answers, but I think we should blame the bad answers, not the question. Look at that close reason again:
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.
Let's focus on that first part, and for us this means:
- Good answers are supported by good historical sources
- Good answers cite the opinions of professional historians, and don't rely on personal speculation
So before you close a question using that reason, consider: do you really know the topic well, and know that there are no historical sources that address the question? Sometimes it's hard to tell a good "why" question from a bad one. Don't conclude that it's a bad one just because it's currently attracting bad answers - we have other tools for that, like the "protect" function. If you're not sure, give it a chance.