There have been several independent ones because they all fail. I don't fully understand the dynamics here, but the inability of the community to create a usefully working OS out of all that open code for over a decade speaks volumes.

I wonder if it is because it never gained enough intertia when Macs are good enough with the Mach kernel etc.? I wonder why the varieties of Darwin didn't catch on?

I think a free software operating system binary-compatible with OSX drivers and source-compatible with OSX applications (through GNUStep) would be an exceedingly compelling product. Which is why people keep trying to make it happen. I don't think it helps that you need XCode to compile a lot of the source that Apple releases, which means you need a Mac.