The book contains so many explicit sexual scenes that are described in detail. Sexual repression, especially among women, led to the fast attraction to the book.
Women think differently about sex. Women need romance, a good story and intense emotions in order to get aroused. 50 shades of grey is perfectly written to arouse the woman's mind. It's like a porn movie that is designed to work for women.
Women love resourceful men and Mr Grey had it all. He was a handsome, famous billionaire and above all, he fell in love with Ana.
50 shades of grey is about the transfer of power. Women don't just love resourceful men, they also love to overpower them. Ana was intimidated by Mr Grey at first but later she had the chance to overpower him. The transfer of power concept is considered very attractive to most women.
Women get attracted to romance. Even though Mr. Grey had problems letting anybody touch his chest because he was abused as a child, he scarified that and endured pain in order to please Ana.
The reason many women love Twilight is because the unattainable seems much more interesting. Just like it seemed impossible to date a vampire, it also seemed impossible to date a perfect man. The impossible love concept added fuel to the fire and made the book even more popular.
Nothing would seem interesting if a woman's friend saw her holding a self-help book, but finding her reading a book that contains some sex stories would intrigue her friend very much. The curiosity sparked by the book helped the word of mouth spread much faster.
Ordinary BDSM books have no plot or story line. They are just written to fulfill a certain purpose. 50 shades of grey is a BDSM book but with a story line. While many people think they just like the story, a part of them wants to read a BDSM book without feeling ashamed.