There has been some interesting discussion of contrastive explanation at Maverick Philosopher and Philosophy, etc.
Are these two questions equivalent?
Q1. Why does anything at all exist?
Q2. Why does anything at all exist (rather than nothing existing at all)?
Now, there are cases where a contrastive question clearly is not equivalent to an apparently similar noncontrastive question. For instance:
P1. Why did the window break?
P2. Why did the window break (rather than crack without breaking)?
The two questions are clearly not equivalent because an adequate answer to P1 will give us information about two things: the conditions required for windows in general to break; the events that fulfilled those conditions in this particular case. An adequate answer to P2, on the other hand also must give us information about the conditions required for windows in general to crack without breaking, and the events that fulfilled those conditions in this particular case. In other words, P2 is a more specific question than P1: to answer it you need not only to know about how windows break; one must also know about how windows crack without breaking. Strictly speaking I think this is true even if we modify P2 in the following way:
P2'. Why did the window break (rather than not break)?
P2' isn't equivalent to P1 (although they are very similar) because P2' is a more specific question: an adequate answer to it requires saying something about what would be involved in the window's not breaking. Of course, it is very tempting to think we can simply assume that the factor that explains the window's breaking is the contradictory of the factor that would have explained the window's not breaking. But we cannot assume that even this is always the case. Consider:
R1. Why did the scanner identify this as blue?
R2. Why did the scanner identify this as blue (rather than not identify it as blue)?
Let's suppose that the reason the scanner identified the object as blue is that it was jammed, so that it would identify anything as blue. But the scanner's not being jammed is not necessarily what would have explained the scanner's not identifying the object as blue. For if the scanner were not jammed it might have identified it as blue anyway. 'The scanner is jammed so that it identifies everything as blue' is an adequate answer to R1. But if the object is blue, and the jammed scanner identifies it as such, the adequate answer to R2 is that 'the scanner would only not identify it as blue if it were off or jammed in some other position'.
Therefore I am inclined to deny the claim that "Why X rather than not-X?" is just the same as "Why X?" The two cannot be equivalent because the conditions for an adequate answer to each are not the same. We can put the point in a crude but clear form. You can answer the latter just by talking about the conditions that actually led to X in this case. To answer the former, you have to talk about the conditions required for not-X as well. But there may be other conditions that would lead to X that have to be taken into account when discussing the latter, that don't have to be taken into account when discussing the former. Q2, P2, and R2 all require some sort of consideration of counterfactuals in the answer. Q1, P1, and R1 do not, because they allow as adequate answers claims that would not be adequate answers to Q2, P2, and R2.
This is not to say that there are no adequate answers to Q2 that would not also be adequate answers to Q1,, or even that all adequate answers to Q2 are adequate answers to Q1 and vice versa; rather, this is to say that the conditions for an adequate answer to Q1 are not the same as the conditions for an adequate answer to Q2. If the same answers are adequate for both questions, they are not adequate for exactly the same reasons. Thus Q1 and Q2 are not equivalent, and we cannot assume on the basis of the question alone that an adequate answer for one will be an adequate answer for the other.
One way to think of it is to think of it in this way. Suppose that things are set up so one factor makes it true in the actual world that something exists; but that there is a 'back-up factor', so that even if the actually explaining factor were to have failed, there would be another fact that would have made it so that something exists. To answer Q1, all we have to do is point to the actually explaining factor. To answer Q2, however, we have to take into account the back-up factor, because leaving it out doesn't explain why anything exists rather than nothing (something would exist even if the actually explaining factor weren't there). But we cannot rule out the possibility of an answer like this simply in virtue of the questions themselves. Therefore Q1 and Q2 are not equivalent, however closely related they might be.