The reason is because of historical oppression and entrenched systems of oppression still in place today. In that context, meritocracy only serves to reinforce the status quo.
I'm sure NASA can come very close to a true meritocracy - but that doesn't change the fact that there are benefits to having women in visible positions of power, accomplishment, etc. There will be multiple qualified people for any job, and it is 100% acceptable to choose someone because they come from a historically underrepresented group, solely to increase visibility and enfranchisement of that group.