Here's why:
* The Bridge is guarded: The bridge is guarded by a powerful trainer named Dardusk, who represents the final challenge before facing the Elite Four.
* The Champion is the key: Dardusk will only allow passage to someone who has proven their strength by becoming the champion of the Sinnoh region.
To get past the Dardusk Bridge, you need to:
1. Defeat the Elite Four: This is the ultimate test of your skills.
2. Become the Champion: After defeating the Elite Four, you'll face the champion of Sinnoh and become the new champion.
3. Return to Dardusk Bridge: Once you're the champion, Dardusk will acknowledge your strength and allow you to pass.
Note: You can try to battle Dardusk before becoming champion, but you won't be able to win. He is designed to be a difficult challenge specifically for the post-game.