I watched an interview with the CEO on the Linux Tech Channel I believe (the french linux guy) and the problem is as usual, that the Linux userbase is too small. Proton, being a fully venture capitalist free company, meaning funded by the users (which is great), has to implement what the majority of users want and those are unfortunately IOS, Android, Windows, and a bit of MacOS. Linux is very far behind those, so comparing the size of the user base vs the amount of features, I'd say Proton isn't doing too badly.

I always recommend Proton Pass. A) because they have a forever free version and B) because hopefully they start looking into the whole suite in general and even if they don't subscribe, they are more aware afterwards (hopefully).