Except that before "just supporting Linux (and maybe MacOS)" was the thing to do, it was "just support Sun (and maybe HP-UX)". And before that, it was "just support BSD/Vax (and maybe SVR2)".
So, when you take a broader view than "just support what everyone uses", you're not just helping niche platforms -- you're future-proofing.
So, when you take a broader view than "just support what everyone uses", you're not just helping niche platforms -- you're future-proofing.