Well, the reasoning behind it is the simple fact that this was done ten years ago: Pleasure in late Spring, Treasure within days of the anniversary. Treasure's songs were then, too, chosen by the fans from what I've read (I believe Keen said it; obviously wasn't around then). While I would love to see ULTRA Treasure be a tremendous success and outsell ULTRA Pleasure, it just doesn't seem too likely given the past. As was said earlier, though: It is possible that Pleasure II will have an effect on this, as it was, after all, nearly half of ULTRA Pleasure and was only released three years ago. So, if that does factor in to slightly lower sales for ULTRA Pleasure in the end, I wouldn't be too surprised. It's best to keep in mind that singles--re-releases or otherwise--always move quite a few units. So far, there are only like seven or eight singles in the top thirty songs for Treasure, so that could factor in a bit as well.
It is a safe bet to say there will be a 2CD+DVD edition of ULTRA Treasure as well, collecting other unreleased performances. A new version of two classics would be a welcome edition, like maybe BLOWIN' or Brotherhood (hey, it's #1, and I know I'd kill for an extended and altered version). Although it's nigh impossible with the original already being on ULTRA Pleasure, the acoustic version of Itsuka would have me interested without question.
I wouldn't count on a 3CD collection, though. That's generally way too many songs for a single release, but that doesn't mean it's impossible. This is B'z, after all.