Because it's open source. It's so ridiculously easy to fork that every time some developer thinks he can do better, he forks.
What I don't get is why all those little teams of developers come together and create on kick ass *insert program here*
Also, a lot of independent developers think they are the only ones who get it right, discarding any critics. The GNOME developers are known for their "regardless of what users want, we know what's best for them" mentality.
Since most open source projects are done either by developers working in their spare time or by companies who do not have truckloads of money, it's not possible for them to test their theories about UX.
There are tons of people at Microsoft getting new ideas on what some software should look or feel like. But they have money, so they pay normal computer users to test their innovations, which means they are able to stop projects that do not work.
In Vista's early days, they actually wanted the window borders to be completely transparent (without blur) so that users could focus on the content. Had they not had testers who quickly told them it was terrible, they might have released it like that.