When I went to download the game, I skipped the donations and only saw the linux biuld. Why is it only free for linux?
It might not work for everyone, and the population of linux users is very small. It is untested by the developer and there is a large amount of linux distros, but there will always only be one MacOs, there will only be one Os for windows, updates might break these versions but that is a very simple fix. It isn't simple to have to update and test the game on every possibly used linux , because that would take an extraordinary amount of time. So it is free for these untested linux distros for people to play because it might not work for everyone, you are taking a large risk if you are paying for a game that you don't know will work or not. Imagine downloading a game on steam, but the hardware it was developed on is very old and the company is now defunct, so it isn't getting any more compatibility updates, you would be taking a large risk for a game that you know won't work. The risk is even bigger for a game that you are unsure won't work.
That is a completely idiotic statement. Sure linux is linux no matter the distro but why do you think there are so many different versions of software made for different distros? Sure some compatibility may be present within these programs but full compatibility is almost impossible, you can try this yourself by copying the command into "ldd /bin/s" into two different distros, most of the time the libraries that will be linked are completely different.