Hiya there!
I think in a lot of cases it comes down to being unable to test the builds for other platforms and to know for sure if they'll run without issue or not. Since unexpected issues might crop up when compiled for those platforms opposed the main one they develop for. I know that's a fear I always have when I push out builds for projects of mine, that technically run on Windows/Mac/Linux, but I have no idea how well--and would hate to potentially mislead someone if it didn't work as intended.