1) Support. Not just a support ticket system but an attitude that its worth investing serious amounts of money/effort to prevent customers from ever losing a message.
2) More functionality. My Fastmail account supports Sieve (mail filtering language, much more powerful than normal rules), CalDAV, and CardDAV. I use Thunderbird (a email client) and am looking forward to replacing my Fastmail IMAP account with a JMAP account when they add support for it (its on their current roadmap).
3) Gmail appears to be the gold standard for spam detection. But most free email accounts seem to do a poor job.
Its hard to envision Google killing or crippling the free version of Gmail but look at the list of the 245 products they've killed off so far in
https://killedbygoogle.com . Some were very popular/successful. Long term you just can't rely upon Google.
4) I don't want to put all my eggs in one basket. Lots of services now rely upon you logging in via either Facebook (I refuse to have an account with them) or Gmail.
5) The possibility of getting your email provided by a company whose only business is email. That avoids many potential conflicts of interest, and focuses their efforts on providing good email service. You can get free email from your ISP but they view it as just a cost center. With Gmail you're not the customer, you're the product.