Here's why:
* Politoed's evolution: Politoed evolves from Poliwhirl using a King's Rock held item.
* King's Rock in FireRed: King's Rock is not obtainable in Pokemon FireRed.
* Generation III Evolution Mechanics: The method of evolving with a held item wasn't introduced until Generation IV (Diamond, Pearl, Platinum).
So, to get Politoed, you'll need to:
1. Trade a Poliwhirl from a newer generation game: This is the easiest way. Trade a Poliwhirl that's holding a King's Rock from a game like Diamond, Pearl, or later.
2. Use a "Generation IV" emulator: If you're okay with emulators, you could play a Generation IV game like Diamond or Pearl, and evolve your Poliwhirl there.
Let me know if you have any other Pokemon questions!