People like it because of the mystical component. Also, most girls dream of a boyfriend like Edward. He's protective, hot, and understanding. The love triangle between Bella, Edward, and Jacob adds to the excitement of the book. I'm pretty sure any girl that had a boyfriend like Edward and have a guy who is in love with them like Jacob would die. lol
Basically it's the love story and the mystical part that people seem to be the selling point.