One redditor asked about it on the most recent AMA:
Hi, is there a plan to release protondrive for linux?
Proton CEO's answer:
We're currently looking at options for how to fund this. It's an expensive development because Linux has so many different flavors and we need deep integration with the filesystem, and it is not yet clear if there are enough Linux users that would allow us to offset the cost of this development. Like many things Linux, it may eventually just have to subsidized from Proton's reserve budget. That doesn't mean it won't get done, it will just take longer since we are also subsidizing several other efforts at this time, such as the Proton VPN free servers for elections campaign: https://protonvpn.com/blog/free-servers-before-elections
-Andy
Source: https://www.reddit.com/r/ProtonMail/comments/1ff211y/comment/lmrdepr
It's unclear that they need to handle a specific flavor when they could release a Flatpak. I think the community wouldn't have any problem tweaking the dependencies for particular distros.
It wouldn't be the distribution method that is challenging, it's the complicated task of monitoring your filesystem for changes, and working with a dozen or so different file systems to do it (the way it's accomplished on an ext4 partition might not work on btrfs, for example).
I'm not skilled enough to be able to speak to that.
Maybe I'm in the minority, but all I need is a fuse plugin. I don't need specific syncing like a OneDrive/Google Drive app.
Why not use rclone? It can mount proton drive.
Flatpak is not the answer here. For example, flatpak version of codium doesn't recognise terminal settings out of the box. Since such trivial thing is a problem, image how difficult it would be to use it with various file systems, sync options, etc.
I agree, the reasoning seems rather outdated. Flatpaks are pretty good.