Because of all accounts, when MJ was living, he was never recognized to have assets over a billion. I've seen his assets valed in different reports over the years from 300-600 million. Now keep in mind, that didn't inlcude the debt that the crazy MJ fans like to pretend he didn't have. If MJ was a billionare, he never would have sold half of neverland ranch. If MJ was a billionare, there never would have been an auction for MJ's personal items, only to be called off once MJ secured a big payday for the London concerts.
I think what some people are confusing here is personal wealth and the amount of money someone generates. I'm sure that MJ has generated over a billion in revenues over his life. But that doesn't make him a billionare becase all; the money is not his. Even in the Martin Bashir interview, MJ didn't say he was a billionare when asked about his money.