Persona 4 was featured in many lists, articles and videos as the best game on Vita, so that contributed greatly to the popularity of the series. But unlike the previous game, Persona 5 will release on PS3 and PS4, which have a far bigger install base that include those who already know how great Persona 4 was but can't get it.
I think popularity is almost synonymous with sales, or can at least mean that you have more potential costumers like in the case of Persona 4.
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