Oh boy, getting old here, let's see if I can remember.

I think in part it was because they seemed unable to fix the Linux desktop client after months and months and months, and repeated support requests. Also, I had been a user of theirs for some time when they changed how their plans worked, and moved a feature I relied heavily upon (I think it was email templates—something you might never need!) to a much more expensive plan. Felt like I was being charged more for being a longtime user. I don't know if they have cloud storage now, but at the time they didn't, and Proton had that, along of course with mail and calendar—all of that for less than what Tuta was offering which was mail, calendar, and plenty of other features that were continuously promised but never released. So really just a bunch of little things that were important to me, but which other users might never miss.