The thing with Disney Villains is that they don't really have enough in common to support a unified "theme" park. A few are alike enough in sensibility that it's fun to see them palling around, but that's not really enough to support a whole park.
People always talk about "Dark Kingdom", but what really would that be? Disney Villains tend to hail from totally different locales, with only a few that are similar enough aesthetically to shove together in a way that doesn't feel completely random. Those are mostly the older ones - Maleficent, Chernabog, The Wicked Queen, Captain Hook, and maybe The Horned King (not that I expect them to acknowledge him at all) could have a cohesive little Villainy Village formed out of their lairs and locations. A hive of skullduggery among rocky outcroppings with crumbling towers and creepy cottages, and other Villains could make appearances within that. But if you tried to bear that out across a whole park you quickly run out of places that both make sense to have in proximity and that guests are really dying to visit.
Other parks shove together disparate locations, but they do it under an overarching theme that's greater than "Imagine if these guys were all neighbors". The lands in MK are culturally aspirational in a way that, like, Cruella's House or Hans' Southern Isles just aren't. The list of strong, popular, and merchandisable Villains with great themed attraction potential really isn't all that long . . . not long enough to support a whole park.
Add to it that Disney hasn't really invested in creating a really good, really bad Villain character in almost 15 years, and it suggests a whole park is not something they could commit to no matter how fun it is to imagine. And it is fun. But a land makes more sense.
Not to mention that it'd be a total odd man out among the given themes for the existing four parks at Walt Disney World - A Magical Kingdom, a Magical Movie Studio, the Natural Realm, Whatever they think EPCOT is anymore, and . . . Mean People?