[Someone]

When x86_64 was introduced, other 64-bits systems had made that choice, too. It was natural to follow that.

See http://www.unix.org/version2/whatsnew/lp64_wp.html and http://www.unix.org/version2/whatsnew/login_64bit.html.

[adrianmsmith]

A counter-example (the only one I’m aware of) is tru64 UNIX from Digital (later Compaq, later HP), now EOL. If you used “int” in C on that platform, it was 64-bit.