Tumblr - Because being a straight up evil villain makes you unlikable. Or something.

Freiza was a remorseless genocidal tyrant, but his dad probably hit him maybe one time so it's not his fault.

Yuda was a heartless murderer and kidnapper, but he was sad cuz he wasn't as fab as Rei so it's alright.

Karz would have subjugated humanity and performed other atrocities, but he saved a puppy and a flower once so leave him alone. That squirrel had it coming tho.

Some of the best villains are sympathetic ones, like Mr. Freeze from Batman or Nox from Wakfu. Unfortunately, I think some people have begun to get the idea that a villain must be sympathetic to be a good villain, which is why they jump through hoops to say "No, no, see, they're just misguided!" They think that complexity and tragedy are required for a good villain.
Besides, the world is easier to take when you believe there is a reason for everything. It's kinda depressing to think that some people are shitty without any real reason. They want to believe that making the world a better place will prevent the creation of evil people. And that's not entirely wrong: fixing poverty and improving education and so on will prevent some people from going down the wrong road and will get some people the help they need. But there's always going to be shitty people. Evil can't always be fixed or prevented, but I can understand why people wouldn't want to believe that.