If you look around your workplace and everyone, or least all the managers, look the same - same sex, skin colour, social class, age - then your company has a diversity problem. But why is it a problem?
Because the most obvious explanation is a failure of meritocracy. Such features as the colour of one's skin or sex are arbitrary and irrelevant to people's ability to do a job. Therefore the fact that people of certain skin colours or sex are missing from your workplace relative to the wider society presents a prima facie challenge to the fairness of your company's criteria for employment and promotion. To assume otherwise - for example that people of certain colours, sex, class, age, happen to have different (inferior) career preferences or different (inferior) talents has no credibility. It is to assume the exact set of facts most convenient to make a problem someone else's, rather than to take responsibility for investigating and fixing it.
Call this the negative argument for diversity: If you don't have internal diversity in line with the wider society then you are probably treating people unfairly and you need to investigate and try to fix it. For example by identifying and mitigating biases in how job applicants are evaluated and structural impediments to their career progress. It leaves a lot of details still to be argued out, but I think it is the right way to go.
But there is another kind of argument that is now much more common, the positive argument that organisations should promote diversity because it pays off. This is the argument I want to criticise, on the grounds that it jeopardises the negative argument from fairness; reduces individuals to stereotypes about groups; and perpetuates unjust stereotypes and social relations.