Historically, villains were almost never more than around 20% of the player population.
So they didn't get a ton of extra content - not enough people played them, so they couldn't justify time and money going to villain-exclusive content.
Which disinclined people from playing villains.
Which meant they got less content.
Which, etc.
It's unfortunate. Villain content tends to be much better-written, but a lot of that is because villains miss all the bad 'KILL X Ys!' stuff and the dozens of contacts who lack arcs and, etc. More recent (relatively speaking) hero content is largely on-par with villain stuff.
Population disparity has been an issue with all MMOs offering multiple sides. Most people want to be the Pretty Side, most people want to be the Good Guys.