Well, for one, they REALLY are pushing this "universal experience" thing, where no country gets a sneak peek at the contents of the game because another country got it first. I get the feeling they want to emulate Gen 1 again, hence the multiple references to it (Kanto starters, Pokemon game montages, etc.). They probably want that "fresh game" feeling from the first games.
Secondly, more hype doesn't always equal more sales. I honestly don't think anyone who was not going to buy the game on release date would suddenly buy it because they learned about something in it; anyone who was waiting to see what was inside before buying was probably going to wait until after the release anyways.
Lastly, it's really just bad practice. If you give someone a gift before Christmas (that you know they've worked on for months), with the tag, "Do Not Open Until Christmas" on it, you probably don't want them to open it before Christmas. Will you open it before Christmas anyways? Probably. But it's one thing to open it; it's another entirely to open it, photograph and videotape its contents, and post it online for everyone (including the gift-giver) to see.