By and large, villains tend to be more interesting than their heroic opposition. The good guys, when you get right down to it, all have the same basic goal: Stop the bad guy from doing whatever bad guy thing they're up to. There's not a lot of room for individuality.
The bad guys, though, are driving the story. And so developers can make them as outlandish and idiosyncratic as they want.
So long as the villain is unique and climactic - and makes the bare minimum of sense - then we gamers will get invested. Because even a memorably bad video game villain is better than one you'll forget half an hour after you finish the final level.
We have no room for generic baddies with boring AI. We want someone special, who either terrifies us, gives us the giggles or just generally excites, one way or the other. And these guys achieved that in spades...