Battle for Undercity got removed in Cata due to the fact that the scenario is completely bugged/broken. It still is in Wrath Classic because they didn’t bother to fix it. If you want to read more:
Battle for Undercity Issues - Updated 10/4 - WoW Classic / Wrath of the Lich King Classic Discussion - World of Warcraft Forums (blizzard.com)
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