They don’t take money from investors but grow organically, which limits their resources quite a bit. With more users being on other platforms and Linux being a bit more complex when it comes to amount of possible filesystem and other combinations I see why it takes them a while. Iirc they also do e2e encryption of (meta-)data which does increase complexity.
Hopefully they’ll finish it at some point, as it’s been a long time since they announced Proton Drive. As I’m not paying for Proton, I understand a paying long-time subscriber might not share my acceptance of them zaking their time.