Maybe they're waiting for enough to demand it?
Anyways, I can see why there are people who are not giving it good reviews:
1. It's the 4th(Yeah, I know about the original and it's PS2 copy) in the series so reviewers can be lazy and just use the same old "not as good as" references they use every single time instead of actually playing the game.
2. It's harder, at least with the vampire. Some people want their games spoon fed to them constantly.
3. Bug, blah, blah, blah. EVERY game has bugs nowadays and the ones for DS I've seen are nothing compared to those. The bugs I've seen have been easily gotten around.
*Playing the Vampire has been hard as hell(I'm playing all 3 at the moment) due to two simple things. Quality of opening creatures(Boy, could I have used a couple more vampires to buy) and lack of spells. I've lost count the number of times I've cast Poison Skull due to simply not having anything else. And this one is randomized as to damage! What's worse, I'm eagerly getting to the next store and finding almost nothing spell wise that helps. At least after Whitehill you can get quality troops from near the Shelter. I lost count the number of spiders and snakes(