reads more like a young adult romance slash fanfiction than anything else.
And that is a large portion of why it's popular. Young adult slash fiction is incredibly prolific for a reason. Stuff that reads like it is very likely to do well if it escapes from the clutches of the rabid yaoi writers and into society at large.
E: Though now I'm suddenly curious if anyone's bothered to try to track the internet's slash fiction output. How many words per day are being released online? Hundreds of thousands? Millions? It would be an interesting statistic to follow. It would be really interesting to see what kind of seasonal etc. variance there is, among a bunch of other stuff. Someone needs to write some kind of program that collects that data.
The big drive is also that it is a HUGE "Beauty and the Beast" where Ana through constant insistence "Saves" and ultimately "Changes" Christian Grey for the better.
Transforming their TERRIBLE BDSM, As in the book depicts BDSM as a flawed system people only do to act out abuse, into loving vanilla.
So you have a large number of sexually repressed people... who even if they were in an erotic situation would probably bow out immediately for multiple reasons.
And you have a book that is basically about someone, who is totally hot and rich and handsome, who forces you to have the pleasure that you even deny yourself... But your not a slut right? So you will eventually turn him away from the dark side and change him for the better, mending his shattered psyche. Ohh and did we mention he was rich?
Believe it or not 50 shades is INCREDIBLY unspecial when it comes to romance novels at least in terms of "Rich Millionaire who is romantic but incredibly mean could have anyone, but he wants you, so you change him for the better"