Quote from base_coat
I think it's popular because it's a reverse harem and perhaps readers find the guys who romance the main girl attractive
I adore it 'cause it's such an utter train wreck. In a weird way, it's the utter unlikableness of the characters that make it compelling. It's very much reading it to see how much worse it can get, but Aihara throws in enough snippets to keep it from becoming completely horrible. So yes, the train wreck/soap opera factor is sometimes very important to shoujo-it's the hook that keeps people reading. Vampire Knight, Nana, Mars and all the other really popular dramas also have this train wreck factor keeping them going.
The other branch of really popular shoujo is the heavily character driven stuff that gets criticized for having horrible pacing and really dense characters. [m]Kimi no Todoke[/m], Koukou Debut, and Lovely Complex are great examples of this and the less popular Akagami no Shirayukihime is a different approach to character heavy stuff, not making anyone dense but still taking everything really slow. I think it leads to really great development, but it also leads to a lot less physical relationship stuff.
Ouran and Perfect Girl Evolution end up coming up somewhere in the middle-lots of character stuff and lots of drama, but kind of too schizophrenic to take an approach. It leads the manga to feel like they're basically gag manga (much more so PGE since Ouran started fleshing out the actual plot the past couple of volumes) which turns off a lot of readers who want more focus.
I think the best shoujo makes use of the fact that it's also both a visual and textual medium-panels that show through the art, strong dialogue, great details, etc.