[norenh]

One reason is that tuta does not require you to have any other connection to create and account. Protonmail require a second mail, phone or possibly some kind of payment if I recall correctly (for verification?) that could be linked from your account in theory.

Without having a good anonymous starting point, protonmail does not let you get that starting point, at least the last time I tired (maybe a year ago).

[PrimeMcFly]

ProtonMail never used to require another email to verify, and only asked for a phone if I was on an IP that had made more than one account already.

[GTP]

Or, AFAIK, if you're registering using TOR.