The three most important factors to consider when creating a successful marketing campaign are exposure, timing, and polish. Even if the story isn’t as good as P3 and the characters aren’t as good as P4, there are some issues with how certain things are portrayed, but Persona 5 is very polished in terms of graphics and gameplay. The story is good overall and the major characters are generally likable and entertaining (although by different degrees).
The series has a good reputation because it pays attention to detail, has a stylish presentation, and is dedicated to telling the stories it wants to. They usually have a lot of content that can be played repeatedly. Persona 4 sold more than 3 games in total, and Persona 3 sold just slightly below the first Persona 2 game. (The worst-selling game was the second Persona game.)
Atlus had a few reissues and at least one port for each of the Persona games, but which ones got localized here was hit or miss, especially with Persona 2.
Because of the active emulation scene and positive reception of those games, word of the games spread and many people tried them out wherever they could.
This created a fervent fanbase eager to get their hands on a new mainline Persona game.
The third factor is the timing and mood of the world in which Persona 5 was released—as well as the surprisingly timely subject matter. Growing global unrest, outrage at corruption and sexual harassment/assault, and the exposure of numerous public figures as not the positive role models people thought they were made Persona 5’s picaresque themes of rebellion and injustice stand out even more.
It was released within weeks or months of multiple scandals
It touched a nerve and got people talking even more.
As a point of trivia, Persona 5 sold over 2.2 million copies, making it the most successful game published by Atlus.