The move from .ch to .com was done in an attempt to deal with the poor search ranking. It was done because the best guess was that Google was "geocoding" Proton

The move from .ch to .com was done in an attempt to deal with the poor search ranking. It was done because the best guess was that Google was "geocoding" ProtonMail as a Swiss-only relevant website and there for ranking it poorly in google.com results. ProtonMail actually did not want to move domains. There was an significant internal struggle over the decision. Up until the change, ProtonMail had actually been promoting the .ch domain as an advantage since the US Gov't/FBI would need to go through the Swiss authorities to seize DNS control of the domain (unlike .com). That is the reason that .ch remains available for customer e-mails.