The reason promoting diversity (e.g. in tech) is good is because there is a shortage of qualified people. If we have certain populations that could work in the field, but don't, for whatever reason, then we want to find those causes and stop them. And if those causes are historical (e.g. past discrimination), we might even go so far as to take that into account.
E.g. take affirmative action for African Americans (in America). Yes, it's partially there to have equal opportunity. But (perhaps more) importantly, we know that because of past diversity, most African Americans have a harder time in education growing up. This probably shows up as people have e.g. lower scores on tests. But what we should be thinking is: "If they got slightly lower scores, but we know that it was harder for them, then imagine what they could do if we help them out with better education!".
One could argue (and other commenters do so) that we should care more about the moral "everyone should have equal opportunities" argument. I agree, but I much prefer to show why diversity tends to be a win-win, not a "you lose so we can win" situation.