Because they want to give the Wii version a chance to sell for a bit?
Um.. I guess that could be true. But this game is very likely only going to be big in Japan. It may have a decent following in the states but no where near what it will have in its home country.
On top of that, I believe that recent sales numbers have shown that the Japanese are crazy for all things Nintendo. Given that (again this is to my understanding, not to be construed as fact) both versions of the game will be able to play together, I don't see why you wouldn't want to build any kind of hype or excitement for your new HD console in your home country.
The game has been out since August 4th, with its first week selling 384,000 units. The second week, it dropped a dramatic 81 percent to 72,000 units. The rush is over. The people that were "super excited had to buy it" already have. It's still number two in the charts, and will probably make respectable sales still. I don't think companies like Nintendo bank on arguments like "We'll hold back the Wii U version so this one can sell for it a bit." Especially when it has a minimum 3 month lead still, and the numbers are trickling as they are. If the momentum kept up at 300,000+ units a week, yeah, I would totally agree.
I don't see the fault in showing off what's coming in the future for your console launching in 3 months. Nintendo can still say "You want to play DQX? Go ahead. It's available right now and you can play with all of your friends online today. But for those of you who want to wait, this is what we have in store."