Proton and Tuta has similar challenges most others don't care about (including FastMail) - End to End Encryption. That itself is a pretty hard nut to crack. FastMail and similar services don't need to think about that, which makes their services simpler.
I would also not claim that Tuta has a quicker development cycle. They had a round recently where more features were highlighted. But that's an exception. I've had a Tuta account for years as well, to test it out, and both the webmail and Android app is still not that feature rich.
And Proton delivers new features and updated apps quite regularly now compared to just a few years ago. Can it be better? Yes, of course. But still, they are doing alot than just 2-3 years ago. And 2-3 years was even better than the years before that.
Also consider that Proton delivers on a broad range of products and services. Mail, Calendar, Drive, Pass and VPN. Tuta basically has Mail and Calendar, where both of these Tuta services being fairly reduced in features still.
My experience (mostly using Mail and a little bit Drive these days) is that Protons releaes are also pretty solid. It's extremely seldom I'm hit by bugs these days. To have that kind of quality requires quite some QA efforts. I'm not claiming the other services are equally good, but Mail and Drive is now very stable - and Mail is especially crucial for my 15-20+ users abd myself.
Finally, Proton serves more than 100 million users by now. Tuta has reached a bit over 10 million, IIRC. That requires Proton to have more staff on support and operations tasks. So even if Proton has more than 400 employees, that's not 400 developers.
@Rookwood @testeronious
Tuta seems to be driven by idealists and privacy activists as well. AFAIK, they also don't have venture capital and their user base of paying users is what keeps them alive. Which is also why it's still a small company.
I don't recall how Tuta got their initial funding to get startet. I don't think they were crowdfunded in the same way Proton did.
But the idealsism goals of both Tuta and Proton is what generally makes it less likely they will sell out.
AFAIR, Skiff was VC funded. The idealism of the founders are easily ignored when the VC backing wants to cash in on their investments. And that's what happened here, in some way or another.