Here's why:
* One-time encounters: The legendary birds are only available for capture once in the game. After you encounter them (even if you don't catch them), they disappear from their locations.
* Post-game changes: The world of Pokémon Yellow changes significantly after you beat the game. While you can still explore the Kanto region, the locations of the legendary birds are not accessible anymore.
However:
* Trading: You can still obtain the legendary birds through trading with other players who have caught them.
* Special events: Some events or special releases of the game might allow you to encounter the legendary birds again, but this is not a regular feature of the original Pokémon Yellow.
So, if you skip catching Zapdos in your playthrough, you won't have another chance to catch it in that specific game.