Here's why:
* Attack Position: Synchro Monsters, like most monsters, can only attack once per turn if they are in Attack Position.
* Special Abilities: Some Synchro Monsters may have unique effects that allow them to attack multiple times under specific conditions. These effects are typically rare and dependent on the specific card.
* Other Cards: Certain cards in Yu-Gi-Oh! can allow monsters to attack twice, but these effects are not inherent to Synchro Monsters.
Example of a Synchro Monster with a unique ability:
* "Shooting Quasar Dragon" can attack twice per turn if it has "Shooting Star Dragon" on the field.
In short, while Synchro Monsters can attack twice under very specific circumstances, it's not a standard rule or a common ability.