I think it's a combination of the multi-year gap between P4/P4G and P5, plus the marketing campaign, plus the chance of a new batch of people wanting to play the whole slice-of-school-life/fighting-evil-by-moonlight style that started with P3. Hell, I remember being excited when it was revealed you were actually fighting and recruiting the old school demons instead of the ink blob Shadows of P3 and P4.
Then again, I feel like something of a heretic for thinking the old P2EP is the best Persona game, and believing it would better to drop the end of the world plots in the post P3 games. I would have been perfectly happy if P4 was a story of a group of superpowered Famous Five style high school students busting a superpowered serial killer, or P5 being the story of superpowered high school students fixing the corruption of society instead of the end of the world stuff.