Because they're using minorities as stunts to get advertising and gain readership. As a minority myself (though I never really understood race as a kid) I was a bit taken aback by that understanding.
It has nothing to do with their stance on inclusiveness and equality.
When Spider-Man was replaced by a half black-half hispanic kid named Miles Morales, that was big news! I was so excited. However, because of the magic of comic books I found out he was only Spider-Man in one specific universe in the Marvel multiverse.
Sorry kid, you're not quite white and nerdy enough
to be Spidey in all the multiverse.
This was Axel Alonso's decision, the editor-in-chief of Marvel. He wanted some diversity nit he comic book industry, he said. At least, the cameras and bloggers picked up on that. In reality, he wanted sales. Profit motive is the chief motivation of any company.
And so we get to the new Thor and the black Captain America.
Unfortunately, in the entire Marvel universe there exists still white male Thor and white male Cap. That doesn't change. And in the advertising, toys, posters, and T-shirts, I guarantee you the white people still stand as the majority.
I would LOVE it if they created Cap as a legacy character. Like DC's The Flash. The guy gets replaced by someone else in cannon, and that person IS the new superhero. They're trying to say that Thor and Cap are specifically this way, but I don't buy it.
Or I guess you can say I don't trust them.