There's a lot to consider when adding in/creating these roles. We have to examine the writers of the scripts and their intended main character's race, and if it is ambiguous, then it's left up to the casting director. Even then, the film director themself may be lookng to avoid a certain look on the screen that some "might not like"
In the end, it's just a giant cash grab, write what will get you the most money, not what's fair. Of course I don't agree with this, and in fact, I believe we could really do well by shaking up the sterotypical white male lead. We're seeing this done more and more in theatre, hopefully it's just a matter of time until we start to see in Hollywood (although cash will remain king in that industry for a while).