They have reviewed all the data carefully and have concluded to a very high level of statistical certainty that the signal they saw could not be explained by random chance, in other words, it can only be caused by the Higgs. There are different levels of statistical certainty thresholds that are used in science, and I think they waited until they could conclusively use a very rigorous threshold to confirm the discovery.
I suspect the Physics Nobel is wrapped up for this year.
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