I think of it like this: open source software is typically written by volunteers. Volunteers with good systems skills but no UX skills can author open source so

I think of it like this: open source software is typically written by volunteers. Volunteers with good systems skills but no UX skills can author open source software on their own. Oppositely, volunteers with no systems skills, only UX skills cannot. This is probably worsened by the fact that people who care about good UX will gravitate towards software with good UX, which is often commercial.