"Why not" sounds like an innocent question but is almost always a criticism. It is worse than a normal criticism BECAUSE it sounds so innocent. So the person uttering those words is passively, subtly attacking the other person who is immediately on the defensive without even necessarily being conscious of this fact. It just feels very uncomfortable and the person doesn't understand why. He was just asked a simple question. And the questioner bears no guilt for being the attacker b/c he just asked for some simple information.
"Why not?"
And the answer is almost never satisfying because it wasn't really a question.
Best to avoid "why not" as it is best in general to avoid criticizing. Criticism usually has for more to do with the criticizer than with the criticized.
"א"ר עקיבא תמה אני אם יש בדור הזה מי שיודע להוכיח" [ערכין ט"ז]
