It's not that it hadn't been hypothesized before, it's rather that the numbers that they attribute to this hypothesis stack up well against a lot of Solar System observations. In other words, the hypothesis is not merely that there is another large, undiscovered planet in the far reaches of the Solar System, but rather the details of its mass and orbit that make this a compelling case. Part of the reason that this hypothesis wasn't possible before is because the observational data to support these numbers was lacking. The amount of knowledge we've gained about the Kuiper Belt over the last decade or so is phenomenal and drives our understanding of what might have happened in the early Solar System. It's worth reading the original paper to get a handle on the rigour with which this hypothesis has been analyzed and the coupling with observations of the Kuiper Belt [1].