It felt right. The original draft of the story had virtually no exposition at all -- the whole thing was like Snape and Luna's conversation -- but then I decided to fill in a few more blanks, and when I started nothing seemed to fit the mood of the fic better than the present tense.

I think it's because it's set in the future, in a wizarding world that's changed significantly in some ways from the one we see in canon, and to use a more immediate tense seemed appropriate to convey the difference.

But I understand it's not necessarily going to work for everyone, just because it is so different from the way JKR writes.