Because Fastmail's sole reason to be in business is to deliver robust email services. It's unlikely that they're going to pull out the service from underneath you at some point when email isn't fashionable.
They also have stellar support.
I switched around a year ago. Another reason is that the interface is faster than google.
Also the features are different, Google has some FM lacks vice versa. Depending on the features you need that might be an advantage or a disadvantage. Personally I only miss "Send & Archive" from Google.
I somehow (not objectiviley) like the impression Fastmail gives me. The cowboy-ish yet serious way they try to compete with the big ones. That might be a reason to join too.