Gregory's Server
Most of open source applications have only one maintainer. So most of them don't have any UX expertise available.
For bigger projects, UX requires a strong unified vision of what the application is going to be. Getting to an agreement on technical issues is hard enough without throwing UX issues in. And forcing volunteers to commit their efforts into a UX they didn't agree with is a terrible idea. Which explains why projects where there are many paid people tend to have better UX.
Getting people to agree on an UX is hard enough in piramidal organizations with tight UX teams. Doing it on big democratic organizations, with many UX experts trying to get their vision catch on must be really, really hard.