Earlier quoted context omitted.
This comes up a lot… The reason is that no OSS projects want a product manager. They want contributors. Most projects, if you say “I think the gui should look like X” they’ll reply with “show me the pull request then”. No one wants a manager when they’re already volunteering, and not enough people who have the skills to make things look aesthetic exist and contribute. Source: look at other comments on this article.
I think the problem is deeper than that: To do a good UI, you need multiple skills: design, development, user testing, etc. It's rare that a single person can do all of that, so you need multiple people to collaborate, which is hard, because of the lack of managers. So you only get contributions from people who can contribute on their own, which is developers.
Probably because for developers, OSS is a badge of honour and can help your CV / find paid jobs - while for designers it's just a dribble nobody cares about.
When designers partecipate, they get to contribute and even impose (sometimes questionable, think kde4) choices.