Here's why:
* The Arceus event was a limited-time event. This event was available for a specific period in the past. It is no longer possible to access it through the original means (Nintendo Wi-Fi Connection).
* The event was distributed via a code. Players received a code that they entered into the game to unlock Arceus.
* You can obtain Arceus through other methods. While the original event is gone, you can still obtain Arceus through other means, such as:
* Trading with other players who already have Arceus.
* Using a save file editor (this is considered a cheat and might affect online gameplay).
If you're looking to get Arceus in your HeartGold game, your best bet is to find a player willing to trade you one.