One of the things MM managed to do was be a quality girl group/set the standard for it. It's why many people (even those in AKB) grew up dreaming of being a Morning Musume member. Tsunku also had a lot to do with it, because he literally was H!P, did songwriting, did production, did all of that.
It's hard though honestly to say exactly what it was that drew me in, but I do kind of know what started drawing me away, i.e. all of the members i was a fan of started just graduating/becoming the old guard and the new ones just didn't interest me that much (that possibly just because i'm getting older now LOL). But i can't even say what it was about the members that were leaving that interested me so much/made me want to support them.