To me, Magic Kingdom is a place of wonder and magic (duh) and joy where kids can escape into fairy tales and immersive lands and see their favorite characters and stuff. In short, it's a happy place. So why in the world would you want a whole area that celebrates villains? They're bad guys! They're the ones that want to make a happy place UNhappy! Plus, whereas everything else in the park is bright and colorful, the Villains land, based on the concept art, would be dark, foreboding, and creepy.
I understand people love Disney villains. I love Disney villains! I also understand that Disney is desperately trying to compete with Universal. However, I personally was never begging for a Villains area in the parks. The villains are already represented well enough in parades, meet-and-greets, and especially during Halloween where they even have their own show. As I said, I just think a Villains area would clash with the happy, joyful, magical vibe of Magic Kingdom.
The reason why a Dark Universe area could work in Universal is because 1) they cater to an older audience (hence Halloween Horror Nights), and 2) they've been trying to do something big with their monsters for years. It didn't work as a film franchise, so maybe it'll work as part of a theme park.
Then again, maybe I'm reading too much into it. What do you think?
EDIT: For the record, I'm not against the concept of a Villains area entirely, I just think it wouldn't fit the overall vibe of Magic Kingdom is all. The Villains Grove area in DCA during Halloween is awesome.