why did they call him wacko jacko

That's just something that got popular. Even once narrator in a 1988 video was calling him Wacko Jacko. Michael didn't like it either , he said in 1997 interview "Where did Wacko come from? I'm Michael Jackson" . I don't like it either.
It's something people who don't like him or are just outsiders use as a point of criticism. It's not really THAT offensive but still it's not something sweet. They kind of call him a weirdo indirectly. I don't think he was a WACKO , he was a perfectly normal human with many ups and downs in his life surrounded by false allegations. He doesn't deserve this.
Anyways , haters gonna hate. After all , he is best known as the "King of Pop Michael Jackson" worldwide instead of "Wacko Jacko" :)