I'm tempted to say, "Because nerdy male gamers will go nuts for anything involving, and indeed focussing on, a unashamedly over-sexualised anime-style girl. Although naturally some of them will attempt to cover this by citing other attractions like "themes" or "depth" or some such."
However, while I am utterly certain that is a factor, it would be simplistic and unfair to leave it at that.
For one, it's made by a bunch called Atlus, who have managed to attract a loyal following, the size of which is unknown to me. I haven't played any of their games, but I've heard interesting things.
For two, it does seem to be making an effort to include the sort of themes rarely, if ever, addressed by video games. I mean, a game about relationships and issues with commitment? That's not something you hear of every day.
For three, some of the rather bizarre imagery has probably attracted some notice. Mostly naked guy clutching a pillow and talking to bipedal sheep? Once again, not something you see often.
Personally, I have little interest in it. The aforementioned themes caught my attention, but what I've seen of the gameplay looks boring and utterly unrelated.