The problem is that the current developer base of many FLOSS project don't understand that a) decent UI requires an expertise and b) knowing how their poor UIs are implemented doesn't count as expertise in UI development.
For example-- if I write to a FLOSS mailing list that the way they are using realloc leaves some stale pointers that can cause crashes, they very quickly fix the problem. But if a UI expert posts something about a similarly severe and easily fixed UI bug, they'll get at least 5 authoritative-sounding responses from people who know absolutely nothing about UI design.
Such a UI developer would definitely need a full-time salary to deal with that kind of social situation.
One additional problem is that it's hard to know who to listen to. Someone who seems like a confident UI expert might as well be a misguided visionary who will turn GIMP into the next Windows 8. Randomly trusting a designer is probably just as dangerous as randomly trusting a software developer.
Compare
"Your UI improvement sounds interesting but it's too risky to trust a random UI expert. Sorry."
To
"I ran the idea by X and they mentioned that accessibility would become a problem with your approach. How do you plan to address that?"
IMO one sounds more likely to attract quality contributions than the other.
I don’t know, but I’ll throw out my theory. Note I’m very much a fan of open source even though this might sound negative. I would also love to see GIMPs UI get an overhaul.
In short, I think it’s harder to prioritize open source features for multiple reasons.
When you don’t have paying customers, it’s very hard to prioritize the feedback you get. When you do have paying customers, you tend to prioritize in proportion to the account size, for better or worse. As a blanket general statement that is not always true, but more true than not, paying customers are trying to get something specific done for their job, while free customers are exploring. I say this from the point of view of owning a web startup with a freemium model, so I’ve had a lot of first hand requests from both paying and free customers. I haven’t had an OSS project with as much success, so there might be some different trends when all users are free users.
The other aspect is that developers of OSS projects have fewer deadlines and can choose what they work on. UI coding is usually less fun than writing filters or architecting an image cache. I know that personally, when I’m left to my own devices, I’m not usually working on the hard or boring things that would have the biggest impact for others, I’m working on the fun things that are most fun for me.
There’s also the issue that OSS projects are created by programmers for programming reasons, and they aren’t often able to hire designers and artists like a for-profit shop.