Age, generally.
Then, some think that newer decks that copy traditional decks in their philosophical or symbolic systems are traditional because they copy a tradition.
Examples of traditional decks: Marseilles, Visconti, Bologna, those sorts of things. Examples of Nouveau-traditional: RWS and Crowley decks, that sort of thing. Examples of tradition-by-imitation: RW-clones, Thoth-clones, that sort of thing.
Examples of non-traditional decks: sundry oracles, animal decks, rune or crystal cards, rainbow-fairy-dolphin-dragon decks. (I don't say they don't do a job, just that they're not traditional).