I used to look at it this way too, but then I thought about it from the filmmaker's point of view. They HAVE to think about these things, such as what ethnicity their characters need to be.
Remember that much of what you'll see of much from the Star Wars universe comes from the films. So if there is never a black person cast, people assume there are no black people in that universe whatsoever. Same with asians or any other particular ethnic group. So when exactly is the right moment for a cast member to be picked as a specific ethnicity?
There has to be a balance of course. You can't just diversify every character just to appear tolerant. You can't just decide to insert a specific ethnic group into a film if it doesn't suit the character, but you're complaining about a character that didn't have a specific look designed for them from the beginning, and as so happens, a black guy got the role.
It's not like they made Han Solo black or Leia/Luke's kid black (as far as we know). It's still a very predominantly white cast, which is fine because the Star Wars movies have been predominantly white anyway. Now, you have one character who happens to be black and everybody is making a big deal out of it.
From the standpoint of a filmmaker, the diversity of your cast should definitely cross your mind because you're creating a film that is representative of an entire universe. So it's up to you to use all the opportunities to show people what kind of people are living in this universe. That means, every so often, casting people of different ethnicity.
Star Wars as a franchise has become a global phenomenon also. So with that in mind, filmmakers would obviously want their film to be representative of a larger demographic.
And what does that have to do with the story anyway? Does it really make a difference if John Boyega's character is black or white? Less relatable maybe?
I think the people behind Star Wars have enough sense to not botch the story.