Without strong leadership and buy-in from everyone involved, it's very hard for design or UX to be involved in open source projects in a meaningful way.
Implementing a design requires discipline and a shared vision, which is often against the bazaar-like nature of open source projects. Everyone's working on their own particular piece and can't agree about anything at all – just look at the internecine battles that are still going on about which init system to use.
In my experience, any attempt at making an interface better, even if it's proven with data and testing, results in extreme pushback about dumbing down. The attitude seems to be that making software accessible is not 'real computing' and therefore should be fought tooth and nail.
Designers often don't work on open source projects not because they don't work for free, but because it's a miserable experience.
Having said that, good counter-examples to this are the recent Blender redesign, where significant effort has been put into making the software more consistent and easier to use, and the Elementary distro where one of the goals is to create a good user experience.