It's clearly a worthwhile investment of resources to their major competitors like Mullvad and IVPN, whose Linux clients are just as good as their Windows/macOS clients, if not better. In contrast, ProtonVPN's Linux app is so painfully buggy to use that you're better off just generating and using Proton Wireguard config files with command-line Wireguard instead.
Wow, I triggered a lot of linux folks. It was just a curious question. And I have another thought, not gonna answer all of you, sorry:
Wouldn't $linuxuser just not simply install something like keypass w/o having to pay for anything? How many services/apps is the average linux user paying for?
I have used linux for ages before I moved to maco. So my bet is: not much. Why then, as a company, would I try to get people to give me money when they are used to getting stuff for free? And would rather tinker with some half-arsed solution (on average) instead of paying $5 per month?
Maybe sentiment has shifted since I was around. Maybe it was the Arch crowd. But my impression was: "I want it free as in free beer and open".
It could be the other way around, their Linux market share is poor due to their poor support of it. Also, the types of people who use Linux also tend to be the types of people who would be interested in their product.