(Bias disclosure: For many years I've been compiling own kernels with embedded filesystem and minimal utilties and then running from removable media, even on the PC, so RPi was easy transition. Further I stopped using disk or swap years ago, even on PC, again making RPi transition painless. Finally, I am more interested in non-graphical tasks so I can live without the use of a GPU.)
The greatest value to me of the RPi is the ability to run a variety of OS. One can readily find other boards that are better than RPi in many ways, but in each case running the variety OS that using a RPi allows for is not as easy, if it is even possible. Despite the disadvantage of RPi's closed bootloader and GPU, the flexibility to run a variety of OS is great advantage over many alternatives, for me.
The way I think about it, user freedom in choosing an OS is actually constrained by "too much choice" in hardware. I reason that this is because it is difficult for OS projects other than Linux to timely add support for each and every different item of hardware coming off the assembly line in each new year. Whereas with the RPi, the "moving target" of hardware support moves less. This IMO opens up possibilities for more OS projects to add support for RPi hardware.
Whether this is the reason a variety of OS are suppported on the RPi I do not know. But I am glad that this variety exists.