Well, I could talk about social justice and basic fairness, but the tone of your question implies that you don't see it that way. So instead, let's look at the question from a coldly calculating point of view.

Let's say I have a open source project, and it's got 100 contributors (or community members, users, whatever -- the it's all the same). If my project follows today's averages, that group will be on the order of 98 men and 2 women. Let's say we work to increase our diversity such that now 50% of our community are women. How big is the community now?

That's right -- 200 people. It's not like adding 50% women removes 50% men... if we increase the number of women in our community, we increase the number of people. Now I've got twice as many contributors, twice as many users, twice as much activity, etc. Open source projects live and die on the strength of their communities. Increasing diversity is often actually the easiest way to increase membership, full stop.

[Now, in practice, I work on diversity because I care about basic fairness. But the point is that even if you don't give a shit about those sorts of ideals, there are still coldly calculating economy-of-scale arguments for why diversity is important.]