Argument to Tradition

Argumentum ad antiquitatem (the argument to antiquity or tradition). Humans generally accept their environment. The "way it's done" may or may not be the best way, even if it were the best way when it became the common practice.

Appeals to tradition are an attempt to set the burden of proof. They can thus be proper or improper, but when the goal of the argument is to show that a practice is reasonable or good, its current state of popularity may be irrelevant.