Here's why:
* Effect Negation is Permanent: When an effect is negated, it's effectively "turned off" for the rest of that turn. This means that even if the conditions for activating the effect are met again, you can't re-activate it.
* "Once Per Turn" Effects: Many effects in Yu-Gi-Oh! have a "Once Per Turn" clause. Even if an effect was negated, that "Once Per Turn" restriction still applies.
Example:
Let's say you activate "Mystical Space Typhoon" to negate an opponent's monster effect. Even if that monster's effect becomes active again later in the turn, you can't use "Mystical Space Typhoon" to negate it again.
Important Note: There are a few exceptions to this rule:
* Some cards specifically allow for re-activation: There are a handful of cards that have effects that can be activated again even if they were negated earlier in the turn. For example, "Sangan" can be Special Summoned from the hand even if its effect was negated.
* "Chain Links" and "Continuous Effects": Some effects are considered "Chain Links" or "Continuous Effects." These effects are not negated in the same way. They might still be active even if they were "negated" during a specific activation.
If you're unsure about whether an effect can be activated again after being negated, it's always best to refer to the card text for clarification.