I think nobody has mentioned the most obvious reason: 90% of open-source projects are used by other developers, not by designers.
Making it hard to contribute can be a barrier, but it's not the number one reason. Designers redesign apps, icons, and sites all the time just for fun, even if they know that their contributions have no chance whatsoever of being used.
Designers want to contribute to something that they care about, and the simple reality is that a majority of open source projects are made to appeal to developers, not designers.
In other words, if you want designers to contribute to your open source project, make it something that they will use and be passionate about.
Making it hard to contribute can be a barrier, but it's not the number one reason. Designers redesign apps, icons, and sites all the time just for fun, even if they know that their contributions have no chance whatsoever of being used.
Designers want to contribute to something that they care about, and the simple reality is that a majority of open source projects are made to appeal to developers, not designers.
In other words, if you want designers to contribute to your open source project, make it something that they will use and be passionate about.