All the reasons have been suggested here. I have been through a similar relationship, she tried going off the pill, we tried all sorts of different things to turn her on in the bedroom, we even tried testosterone treatment and although I found out later that she had slept with a few other men on binges she didn't have a full on affair. I am definitely not crap in bed nor am I selfish or a premature cummer, she was just not interested in sex, period. I had even learnt post separation from the wife of her brother that the asexual problem seemed to run in the family....
I wasted far too much of my life with this woman, I'm no sex maniac but I do believe that sex is the foundation of 99.9% of every intimate relationship, if you are in such a relationship and you have given what you think is your best effort at making it work and it isn't working for you then it's time to move on.