I rephrased my comment - I think it's the cleaner codebase that makes the big difference. Its BSD license has some perks, though - for one thing, it's part of the OpenBSD base system now, which means security auditing and excellent maintainers. That will probably also lead to more features in the long run.

OpenBSD man page: http://www.openbsd.org/cgi-bin/man.cgi?query=tmux