For the same reason that Novartis' everolimus trial they had an abstract on at ESMO in which they said it "didn't meet the trials endpoints" yet the trial showed substantial efficacy. A trial can show efficacy, yet still not meet the pre-determined endpoint.
I don't know what Merck's endpoint is. Neither do you.
What I do know is that it doubled the PFS, and it doubled the response rate.
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