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.