My guess is it's because Google's security team is top notch and happy to share their threat intel with their users and that's much more relevant to journalists than using a non-US based service or one that avoids content based ad serving.
Except support is nonexistent, so if anything happens to your gmail access you're screwed there's nobody to contact. I would think any non profit who relies on emails for fundraising/networking would want a paid service like FastMail or other paid service with 2FA
Only in the alternate timeline where customer support and nerd optics are more important than platform security, which nobody other than Google does better.
Until you get locked out of your account of course, for a number of reasons such as heavy use that triggers an arbitrary lockout and there's nobody to contact. Somebody might also wish to purposely DoS your account with incorrect logins for a number of reasons too, like hiding a money transfer notification or just to screw with you.
So pay for G-Suite, the support is excellent. Non-profits can get it for free.