Here's why:
* Banned and Limited Lists: Yu-Gi-Oh! has a constantly evolving Forbidden & Limited List. This list dictates which cards are banned, limited, or semi-limited, meaning they cannot be used, have a limited number of copies allowed, or have a limited number of copies allowed in a deck, respectively.
* Format Regulations: Tournaments are usually held in a specific format, such as "TCG Advanced Format," which restricts the use of cards not printed in the official English TCG.
* Card Legality: Certain older cards may have been errata'd (changed) or have entirely different effects than their counterparts in the current TCG. Using these old cards in a tournament could lead to confusion and unfair play.
In short, you can only use cards that are legal in the current tournament format and are printed in the official English TCG.
You can find the latest Forbidden & Limited List and tournament format information on the official Yu-Gi-Oh! website.