For the 80% of leading actors being white. Well Hollywood isn't really an American phenomenon anymore. Chances are that a sizeable chunk of those actors are actually Canadian.

If you think of any leading male white actor in the past 20 years, chances are that they are Canadian.

Jim Carrey? Canadian...

Keanu Reeves? Canadian...

Ryan Gosling? Canadian...

Ryan Reynolds? Canadian...

Even if you go further afield, guys like Leslie Nielsen, Wiliam Shatner...etc. were Canadian...

Hell, even Justin Bieber is Canadian...

Furthermore, a large percentage of "Hollywood" movies are not actually shot in Hollywood, or even the US for that matter. Canada is one of the leading places where Hollywood movies are shot. A lot of other movies are shot in places like Prague or Budapest. Plus you have to keep in mind that it's not only American audiences that are watching these movies, but they actually get more money from their international audiences. That's why they have started pandering to the Chinese market in recent years.

And that's what it comes down to in Hollywood... MONEY... The producers would put in a Martian for all they care in a leading role, only if they make them money. Guys like Eddie Murphy, Will Smith, or Denzel Washington had international appeal and that's why they got cast in leading roles. If other black actors come along and they have this type of appeal, they will get cast.