Because being exclusive on the grounds of race, gender or age is against the law? All of those are illegal in the UK where the speakers are based.
More diversity in the workplace also makes them more enjoyable places to work. They may also bring different experiences and points of view in how a site or piece of software may be most useful, leading to more success in the market. A market where women occasionally buy things too.
Same for conference speakers. I want to hear perspectives that I cannot have had. There'd be no point going if everyone had my white middle aged male point of view. I want to understand and see the opportunities I'd otherwise miss, the UX, marketing or expectation differences between generations, gender or regions.
I'm surprised we still have this conversation, sadly.