Because it's a programming language we're talking about, not a political party.
Categorically wrong. This is a programming language community that we're talking about. Haskell hasn't dropped out of thin air. It's the output of a mostly-voluntary community effort. To strengthen the community by making it open to people who have different backgrounds is to strengthen the language.
The real question is why do you only see racial diversity as a problem.
My question to you would be why did you interpret that person's comment that way? Rejecting the hyper-focus on racial diversity which is common in some parts of progressive Western culture is not at all the same as seeing racial diversity as a problem. I agree that in a broad sense a community is strengthened by diverse membership, but I don't agree that the White Guilt hinted at in the original article is constructive.
I think you've constructed a straw-man to argue against here, and that's not at a very nice thing to do.
That said, it wasn't the point of the article. It's fast becoming the point of this comment thread, though, which is unfortunate.
I would very much like for you to explain to me how everything you read is about racial diversity, when so little of what I read seems to be.