Here's why:
* Field Spell Card Effect: The effect of Field Spell cards usually states they are activated in the Spell & Trap Zone and remain active for the duration of the duel.
* Only One Active Field Spell: The game rules state that a player can only have one Field Spell card active at a time. If you try to activate a second Field Spell while one is already active, it will not be allowed.
However, there are some exceptions:
* Specific Card Effects: Some cards may have effects that allow for multiple Field Spells to be active. For example, the card "Sky Scourge of the Sky Fortress" allows you to activate a second Field Spell if you control "Sky Scourge of the Sky Fortress".
* "Double Field" Effects: Some cards, like "Double Field", can activate another Field Spell temporarily. However, this effect is only temporary, and the player will only have one active Field Spell once the effect ends.
In conclusion, generally, each player can only have one Field Spell active at a time, unless a card effect allows for an exception.