It's just possible that's exactly it, why answer all these questions in emails right now when the GDC is almost here, and part of him topping last year's presentation is that with all these fan questions he can tell people exactly what they want to know?
Or maybe it's just that they know every tidbit that people get emailed with will instantly become "locked" info, and if they answered all these questions at this point they might actually paint themselves into a corner. The result would be cutting features that they "promised", because it was later realized it wasn't feasible or fun, and suddenly the hype turns on them.
And, if they give us all the breadcrumbs now they've got nothing to string us along until the thing's finished.