Ad antiquitatem

X has always been done. Therefore X is right.

Claim something to be well-established and proven. Say that it is traditional, and that to change it would be sacrilegious or very wrong in some way.

Tradition, once established, becomes a cultural thing, where people do it without thinking and defend it simply because it now is a part of the woodwork. Familiarity breeds both ignorance of the true value of something and a reluctance to give up the 'tried and true'.