I think Persona, and the whole SMT subset for that matter, deals with imagery and issues that aren't apt to western mainstream audiences, plus it's obvious anime/manga style, and the perceived passe turn-based system will keep it tethered. FF is really the only game series that seems able to get away with that on a wide scale, probably because it's been so established for so long (and that series has already gotten away from turn-based combat).
I don't think Souls is the best analogy honestly. It's gameplay and style is a lot more western friendly, even if its storytelling and difficulty curve isn't so much. But I think the former helped foster it to evolve past its niche.
I mean, don't get me wrong or anything, I've grown to really love the Persona series and all, but I don't really think it's in a position to break out of that niche, at least not to the degree that some people seem to think it can. But I could be wrong, and I'll be glad to. I just personally don't think it will happen.