While there are most likely somewhat racist/sexist/bigot intents at times with Hollywood, I want to offer a different point that could be a bigger problem overall, and that is just the need to use big named people in a movie in order for it to sell.
These days it seems that when a big blockbuster comes out, whenever a preview is shown, there seems to be an emphasis on who the actors are. When seeing previews for Lucy, it really seemed to home in on the idea of "Hey, this is the person who played Black Widow in the Avengers. You all loved that, so you will love this movie." The problem can extend then extend to movies with minority leads as well. If you are going to have an action movie with a person of a color other than white, they will most likely choose from an even smaller pool of well known people. Heck, look at something like Dolphin Tale, which could've had any actor, but chose Morgan Freeman for some reason.
People seem to like to go to movies with actors they know and like, and with Hollywood knowing that, instead of trying to find people to fit the role, they take a famous person, have them in it, and use their name to sell the movie. This makes a select number of people always appearing, while others only take small side roles, or only star in lesser known movies. Thus we get Johanneson playing the character, because she can get people in, unlike if you got any other person, because that wouldn't sell tickets. This also probably shows they aren't confident in their project, so need big names to really sell.
In the end, something needs to happen where the idea of only well known people staring in the big movies has to go away. This has to go with people not caring about acting choices, as well as previews selling the actual product, not the people selling the movie because they are famous.