Well, Black Bolt had just vaporized an entire city with his voice moments before that conflict. His voice is unbelievably powerful and destructive, but it draws its energy from the same electron charge that empowers him in other ways, absorbed through the tuning fork on his head. Having expended so much energy in destroying Attilan (and surviving the detonation), there likely wasn’t as much gas left in the tank for the subsequent fight with Thanos.
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