The reality is those other platforms won’t make much difference on sales at all, and with a limited indie dev team they’ve made a wise decision to focus on the largest PC storefront.
It’s the same reason a lot of indies don’t target Linux, the effort vs reward simply doesn’t make sense for small teams. Anyone who says “But Unity and Unreal Engine support Linux! It’s literally two clicks!” has no idea what they’re talking about and hasn’t actually been through the process of releasing a game for multiple platforms.