Here's why:
* The King's Rock is not obtainable in Pokemon Sapphire. It is only available in Pokemon Ruby and Emerald.
* The King's Rock is used to evolve certain Pokemon. It is specifically needed to evolve Poliwhirl into Politoed and Slowpoke into Slowking.
If you're playing Pokemon Sapphire, you'll need to trade with someone who has Pokemon Ruby or Emerald to get a King's Rock.