Because I don't think it's vestigial, I think it's serving an important function of governance that never made it into the official rules but is nonetheless necessary as a stabilizing effect. It doesn't have to be the filibuster but something ought to provide the effect. It should be easier to block legislation than to pass it. It wouldn't be a good thing if you could have huge policy swings when a 51-49 becomes 49-51. Being able to, with effort, demand specific pieces of legislation reach a higher bar biases us toward the status quo.