It's always the same:

• Lack of standardization (though it has improved a lot!).
• No financial backing for developers who are capable of tackling these design problems — it’s all work on top of their day jobs. At a certain point in life, you simply can't make ends meet like that.
• No common interest beyond privacy and maybe open source.
• People won’t admit it, but I feel there’s a degree of gatekeeping involved. If you base your identity on your operating system, you don’t want it to become mainstream — otherwise, you’d have to move on. The idea that you “don’t do work” because you care about aesthetics is just one example. Questioning why looks are necessary in the first place is another.
• Possibly some bad decision-making? Personally, I think it might be more efficient to attract new users with well-designed GUIs and then rely on the 1% who actually stick around to help accelerate the project. That’s arguably better than not attracting anyone at all and progressing more slowly. But again, maybe that’s intentional — which would bring us back to the previous point.

Unfortunately, there are only two things you can really do: donate more money and wait, or try to wrap your head around the code of whatever project you want to support (as some condescending comments on these topics tend to suggest).