Because it's a disproportionate amount of effort to natively support an extra OS (particularly one as fragmented as Linux), especially one with such a small userbase that largely isn't interested in using proprietary cloud services in the first place because of data privacy and security concerns.
Obviously not all Linux users are super worried about that stuff (I mean, I use Linux and have a google pixel), but on average the Linux userbase is way way more aware of that stuff than most users who just want their photos backed up without having to worry about it.