This phrase extends beyond gaming, with tradition being a logical fallacy. But it does get referenced a heck of a lot.
The claim of “because tradition” is the justification for rules and mechanics that exist in their current form because they have always existed in a similar form. They’re design that is grandfathered in to the system. A lot of people really dislike these legacy mechanics, referring to them as “sacred cows”. While I’m okaying with fixing broken rules, I’m not okay with going too far away from tradition. There is no shortage of awesome games out at the moment – especially with Kickstarter – so tradition is very much the only appeal some games have. If you change too much of foundation of D&D or Pathfinder then it’s not really the same game anymore.