In Pokemon soulsilver why did steven leave the museum?

Steven doesn't leave the museum in Pokémon SoulSilver (or any other Pokémon game). He's the museum's curator and remains there throughout the game.

It's possible you're thinking of a different character or a different event. Could you please provide more context? Perhaps you are thinking of:

* Wallace: The Gym Leader of Slateport City, who leaves his position to become the Champion after you defeat him.

* A specific event in the game: For example, perhaps Steven is temporarily absent from the museum for a quest or event.

Please tell me more about what you're remembering so I can help you further!