I agree that "increase diversity" is a rather obtuse meta argument, and using it implicitly argues that there are not other significant discrimination issues facing these groups. Maybe it is a better argument to make in certain settings though. For example, a company wouldn't want to admit to having discriminatory biases in hiring, so "increase diversity" is a much more palatable objective.

The more honest, less PR answer is that people still discriminate on huge range of factors. Race and Gender are just the most obvious and egregious.