Sales can often be attributed to how good or bad a game is.
But even good games can struggle to find an audience. (And I'm not suggesting Concord is good btw.)
At VGChartz we have the original Persona 3's sales listed at 0.45m units worldwide.
While Persona 5 is selling many times more than that. (The total combined sales between all versions surpasses 10 million.)
Is Persona 5 that much better of a game than Persona 3 that it went from being niche to very popular?
Not really. But P5 managed to market itself better.
Same can be said for franchises like Nier and Yakuza. Their audience suddenly grew many times over with Nier: Automata and Yakuza 0, even though they didn't really change their formula.