The servers and the company that owns them are in Switzerland outside U.S. jurisdiction, a big plus these days.
Some argue that hosting your email on your own server is crucial for true security, but self-hosting also makes it inherently difficult to protect against certain vulnerabilities. Jurisdiction -- and the potential for government-sanctioned seizures or information release -- is an example of that kind of vulnerability. I suppose if you could find a Swiss VPS provider with no U.S. ties you could self-host that way, or failing that you could co-locate in a Swiss-owned property in Switzerland. For those without strong sysadmin skills and lots of money, ProtonMail makes the benefits of Swiss jurisdiction infinitely more feasible with almost no drawbacks.