It was a few years ago so I don’t fully recall. It certainly wasn’t because I had a problem with FastMail. I think it was something along the lines of:
- Cheaper for the price point I was looking for
- I only needed e-mail, not calendar or notes support
- I was attracted by their privacy focus and eco credentials
- They have good technical documentation (not that FM doesn’t)
All in, I would recommend both services readily to be honest.