Hello! We're so happy you've enjoyed the games, and thank you so much for playing. As for why the Steam versions do not support Mac, that is because several years ago, Apple changed the their approval process for games, requiring lengthy review and fees to determine if a game was "safe" for their platform. Itch.io is fine with apps that haven't does this process, but Steam requires it. It's possible some day we might go through the effort, but we aren't sure when that will be.
However, if you can still download the free Mac version from itch.io, the Steam version of the DLCs is cross-compatible, and so you can play them on Mac without having to purchase anything on itch.io itself. We can potentially help walk you through the process if you need help. Either way, we appreciate you being here, and hope you have a wonderful day!