To me, "how" is the physical reason for something, while "why" is either an explanation of the process or, when not physical, the rational or motivation or something similar.
In relation to discussions here on the site, the following certainly doesn't apply to all theists, but we have two here now who are immersed in some dead end conversations because it applies to them.
When they ask "Why?", the aren't actually curious. They always have their answer, with or without a question. And when they do ask, they make sure to word their query in such a way that "God" is the only possible answer.
With that attitude, the only real curiosity they ever display is when they ask "Why doesn't everyone believe me?" Because obviously no god is involved in heretical thought. It must feel weird to them to not have a satisfactory answer to that one aspect of life that they can't pretend to control.
Just to be absolutely clear, I am not including Mooby (or Old Church Guy) in this group. Unless they want to volunteer.