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Several times, I've seen 'why' questions dismissed unfairly when a better answer could be given, because of a kneejerk reaction that a 'why' question can never have an answer, because physics is built to model reality. Below are some examples.

  • A questions about Ito and Stratonovich calculus, asking why we choose one over the other in physics. There are lots of disparaging comments about how such a question is meaningless.
  • A really deep question about QCD bound states, which received an answer saying "physics does not answer why questions".
  • A question about SU(4) symmetry, which is being received poorly.
  • A question about the spin of fundamental particles, which I can totally imagine being received poorly today, but happened to get a fantastic answer when it was asked.

As shown by the last example, there are lots of good possible answers to a 'why X' question.

  • You can show that if X weren't true, there would be some internal contradiction, or you would lose some important, more fundamental principle.
  • You can show by analogy with simpler theories (e.g. classical limit) that X makes sense. Or, if there are deeper theories, you can derive X from one of them.
  • You can use anthropics, i.e. argue that something would go horribly wrong with the universe if X weren't true.

None of these things answer "why" in the philosophical sense, but they do answer it perfectly well in the physical sense. Can we just assume that question posters mean the latter, instead of ridiculing them for possibly meaning the former?

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    $\begingroup$ Because too many Feynman fans have seen this video: youtube.com/watch?v=qjmtJpzoW0o, and have taken it too far. $\endgroup$ – Kenshin Apr 2 '16 at 4:39
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    $\begingroup$ It's worth noting that the question "Why?" is pretty much the driving force in science, and makes humans different from every other animal. $\endgroup$ – corsiKa Apr 12 '16 at 19:22
  • $\begingroup$ New link to the above mentioned Feynman Magnets video. The old link is not working. Link - youtube.com/watch?v=MO0r930Sn_8 $\endgroup$ – Apoorv Potnis Oct 31 '17 at 17:38
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Not all "Why X?" questions are created equal.

The appearance of the word "why?" alone tells you nothing about a question. Let's go through your examples first to see explicitly what I mean:

  • The question title Itô or Stratonovich calculus: which one is more relevant from the point of view of physics? is badly chosen. The actual question seems to be: "How do I decide when to use Itô and when to use Stratonovich calculus in physics?", but the title is what people first see (and based on which, I will admit, I first thought this was a primarily opinion-based question). The "lots of disparaging comments" are all from one user, that is hardly representative of any community viewpoint. I don't see why you would label this a "why" question at all.

  • The "really deep question" about QCD bound states and fractional charges is well-received (9 upvotes, no downvotes), and has no answer in the Standard Model (the Rishon model in the other answer is currently not believed to be realized in nature, as far as I know). The answer that "this is what we experimentally find, there is no deeper reason" is a perfectly fine answer from the currently accepted viewpoint, I don't know what irks you about it.

  • The poorly received question about SU(4) symmetry was rightfully poorly received. It shows no awareness of how predictions of a SU(4) theory would differ from what we experimentally observe, and it is not clear what sort of "why" it asks for: "Why" is the SU(4) not part of the Standard Model? (Because we don't observe it, duh!) "Why" is an SU(4) not possible in QFT? (Meaningless question, as it is possible) The edit asks: "Can the Standard Model be extended in a straightfoward way to include an SU(4) gauge field?" which is a much better and more precise question!

  • The question about spin-2 is indeed a good one, and I would see it well-received "even" today. That is because it is not a vague "Why don't we have spin-2?" type of question, but it specifically asks about the reason one cannot construct consistent interacting QFTs with higher spin. It's a fine question.

Altogether, I dispute your claim that "Why X?" questions are poorly received.

What is poorly received are "Why X?" questions that additionally happen to ask about nothing specific, or that seem non-sensical. For instance, "Why is X not part of the Standard Model?" always has the trivial answer "because we don't observe what X predicts.", and is a boring question and should be poorly received. "Why is there no consistent theory with X?", "Is there an extension of the Standard Model with X?" or "Is X possible in ?" are perfectly fine questions.

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    $\begingroup$ The answer to "Why is X not part of the standard model?" might be "because we don't observe what X predicts --- but it might also be "because X would lead to a logical contradiction within the standard model". It seems to me (in principle at least --- I'm not thinking of a specific example) that it might be quite interesting to ask, in effect, "Would X contradict the standard model?" and that a reasonable way of phrasing that question might be "Why is X not part of the standard model?" $\endgroup$ – WillO Apr 9 '16 at 21:32
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I agree with a comment from @Mew: Feynman's legendary tetchy response to a 'why' question (why magnets repel) from an interviewer made an impression on many physicists. Feynman insisted that whilst it was an excellent question, it was also problematic, as any answer might precipitate a further question, and, depending on the interviewer's knowledge of physics, be incomplete or dishonest.

Feynman's reaction to 'why' questions has become something of a platitude that, when invoked inappropriately, closes the door on many reasonable and scientific lines of thought and questions. Unfortunately, the hagiography surrounding Feynman means that it's likely to stick around.

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    $\begingroup$ Many questions with the word "Why" in them that aren't actually rotten questions can be useful re-phrased as "How does [phenomena in a high level theory] arise from [theory one level lower]" and not trip the sometimes simple-minded reaction to the word. $\endgroup$ – dmckee Apr 8 '16 at 3:35

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