BV claims that the question "Why is it that P rather than ~P?" presupposes that ~P is possible. This strikes me as mistaken. The question could be perfectly well answered with: "because ~P is impossible."
Contra BV, I think the question "Why is it that P?" (and, for that matter, "Why isn't it that ~P?") are clearly equivalent to the above question. To ask why something is, is to ask why it isn't not. The question is always implicitly contrastive in this broad sense. Another way to put it is that the contrastive version is redundant: to ask "why P rather than not ~P?" asks nothing more beyond "why P?".
See also: Why does the universe exist?