Of all tunes available in the world, why do we choose a "worker's" song? A song composed by slaves as they served their masters?
Why not something more uplifting?
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We are thanking Hashem that Haman wanted to kill us. Had he only made us slaves, Esther wouldn't have gone to speak to Achashveirosh and we would have remained slaves.
To remind us what could have happened, we sing a slave song.