I tried reading that fic a few months back. I didn't get near finishing it. It wasn't horrifically bad like some (okay, nearly all) indy/bashing/harem stories are. But neither was there anything to compel me to keep reading. It just... bored me.

 

Yes because its wish fulfilment. It's the ultimate male wish fulfillment fantasy. Who wouldn't their own Harem?

Also another reason is indecisiveness. Hermione is the clever one, Luna is the nice one, Ginny is the sporty one, Fleur is the real babe, Gabrielle is mini Fleur, Daphne is the quiet one, Susan is the kind and caring one, Tonks is the fun one. How do you choose? Why choose when you can have them all.

I also think another subset of Harems is more realistic. Sex slaves. It's more believable women are forced brainwashed to be in an Harem than willingly be one.