50 Shades was originally Twilight fan fiction. So it’s not crazy that 50 Shades took off as well.
And while Twilight was driven by teenagers, myself included, I remembered there being a REALLY strong and outspoken group of older women who were obsessed with Edward.
I also think it’s because a lot of people identify with the heroine. Most of us feel completely ordinary, nothing really special about us- like Anastasia and Bella Sawn. And the idea that a broken billionaire or vampire would be willing to break all the rules for these women is an enticing dream/ fantasy.
I agree with one of the above comments- WHY Twilight and 50 Shades- when there’s so many of these same tropes over and over again? But then I would argue why NOT them? Some book has to do it.