The short answer is that what CERN has observed is consistent with the existence of a Higgs boson but that the number of candidate ‘Higgs events’ falls short of the number required to announce the result with more than about 50% certainty. A slightly longer answer is that experiments like ATLAS and CMS produce results that are ambiguous. The particle is highly unstable lasting only a fraction of a second before it decays. The experiments cannot see the particle directly, they can only see the debris left when the particle decays. A single event has multiple explanations. The decay of a Higgs particle might have given rise to a particular set of tracks but so might many other processes. The only way to be sure is to perform the experiment over and over again until there are enough events to distinguish statistically. — This, this and this