Why the Confederates left Harpers Ferry?

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   I am currently reading the biography of Robert Lee by Burke Davis and it says that  when the war erupted, the Southerners occupied Harpers Ferry under the command of TJ Jackson, but then Jackson was replaced by Johnston and then Johnston abandoned Harpers Ferry and shifted his troops South. Why did he do that? I know that Harpers Ferry was the armory of the North, it was a vital settlement to hold. 