I'll add another potential reason - culture. Many (most) developers grew around open source, or at least interacted with it in significant ways during their (work and/or hobby) career. Many if not most of the tools they (we) use for programming come from open source too. So we're kind of used to this environment of collaboration. You see a project you like and have some spare time, it's natural to contribute.

I doubt UX specialists live in such a culture. I might be wrong, but from my experience with artists and designers, it's much more of an individualistic culture with stronger commercial interest. I.e. "I do it for myself" and then "want my work - pay me".

If that is true, then there's simply much less UX specialists with spare time and interest in open source than there are such developers.