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cus that was the major battle, and the elves from the begining didnt not want to be a major deciding force in the war for the one ring. that is why.
But hten they changed their minds. That was the whole point of that scene with galadrial and Elrond in TTT. And Galadriels men going to Helms deep. And Arwen convincing Elrond to reforge the sword and take it to Aragorn.
The elves changed their minds, and decided not to abandon the men and leave them to their fates. The opted to be pro-active in the conflict, instead of casual observers.