I don't work on open source but it could be:
- Good design requires user interviews, which are less common in open source - The sorts of people who are most attracted to open source work are programmers, not designers - Open source software can get popular even with bad design; closed source software with bad design fails and shuts down
Designers have it really hard in free software because good design is about understanding the big picture, knowing what to leave out, have product vision to follow. That's tough to do in environment where goal is to make everyone happy and the biggest say have programmers who are developing the thing who are usually not interested in design, inexperienced in design or have their personal very specific product vision.