Here's why:
* Pokerus doubles the speed of your Pokémon's EV training. This means your Pokémon gains twice the Effort Values (EVs) for each battle.
* Pokerus is a virus that eventually goes away. It's cured by resting at a Pokémon Center or using a certain item.
* Once cured, your Pokémon will no longer gain EVs at twice the speed. The effect is temporary and does not linger after the virus is gone.
Therefore, the effect of Pokerus does not stay in effect after it is cured. Your Pokémon will revert to its normal EV training rate.