They're located in a country with relatively good privacy laws, they're big enough that I trust them to remain alive for the foreseeable future, they have relatively good uptime compared to other email providers, they're popular enough that they don't tend to land in spam filters, and their corporate structure makes them more immune to enshittification than other companies.

On the minus side, the way their email is set up means you can't easily use third-party email clients without using an intermediary tool (Proton Mail Bridge), and because email is encrypted at rest, their own email search is abysmal.