This is because they are from different formats and are not compatible with each other. Here's why:
* Different card types: 5D's cards feature unique mechanics like Synchro Summoning and Tuning, which are not compatible with the traditional summoning methods of the original Yu-Gi-Oh! format.
* Different rules: The 5D's format has its own specific rules and card effects that differ from the original game.
* Different card designs: 5D's cards have a distinct design aesthetic and card frame, making them easily distinguishable from traditional Yu-Gi-Oh! cards.
In summary, using 5D's cards with regular Yu-Gi-Oh! cards is not allowed and would be considered an illegal deck. You can only play with cards from the same format and era.