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