Some programmers passionately believe that contributing to OSS is status-increasing, perhaps enough to be a self-fulfilling prophecy. I have yet to meet a designer who believes this.
There exist UX paths by which contributing to OSS via code is easy, obviously wanted, and generates warm fuzzies. The fork-and-go experience of adding one marginal feature to a gem for Ruby then sending a pull request feels like ponies and sunshine every step of the way. By contrast, there exists no infrastructure or UX to support designers contributing in the same way. You probably have to send an email to a project maintainer. Try sending ten of those: report how sunny your ponies feel as a result.
OSS projects, in the main, very aggressively signal that they do not care about design. Just having a logo makes it look like you have super-anomalous levels of care relative to OSS. Designers pick up on hints like "your web presence is a default Github page" and "your logo was created by a programmer in MsPaint" to decide that their talents are probably better applied elsewhere, where they will be valued.
Also, 99designs will pay more designers more money today than entire communities of OSS developers will this year. Even starving artists can do the math.
I think just as large a part is that it's difficult for multiple designers to contribute to a project in a meaningful way without stepping on each others' toes. Do designers often collaborate in groups of more than two? (Layer Tennis is the only thing that comes to mind)
If I were evaluating a project I would choose a project with a default Github page over a project done in MSPaint just because there's a good chance that the latter was written by a developer who likes the MSPaint logo.
There exist UX paths by which contributing to OSS via code is easy, obviously wanted, and generates warm fuzzies. The fork-and-go experience of adding one marginal feature to a gem for Ruby then sending a pull request feels like ponies and sunshine every step of the way. By contrast, there exists no infrastructure or UX to support designers contributing in the same way. You probably have to send an email to a project maintainer. Try sending ten of those: report how sunny your ponies feel as a result.
OSS projects, in the main, very aggressively signal that they do not care about design. Just having a logo makes it look like you have super-anomalous levels of care relative to OSS. Designers pick up on hints like "your web presence is a default Github page" and "your logo was created by a programmer in MsPaint" to decide that their talents are probably better applied elsewhere, where they will be valued.
Also, 99designs will pay more designers more money today than entire communities of OSS developers will this year. Even starving artists can do the math.