The Ayu versus Utada topic is so exhausting. There are just too many variables so it is impossible to say exactly why Utada has a more positive image and sells more etc.
I think Utada just has a broader appeal and Fantome seems to support that. Being fluent in English and releasing English songs has given her exposure to the rest of the world. This gives her bigger status in the industry and listeners eyes.
Utada's style of music is also something that benefits her. I may be going out on a limb here but, Utada's music doesn't really have a Jpop sound to me. It's Jpop in the sense that the lyrics are in Japanese and it was created and released in Japan. But Utada has a style all her own within each album. Exceptions can be made for First Love and Distance since they are a bit more straightforward with a defined R&B influence. Plus some pop rock on Distance. Ayu's music is very Japanese in contrast. It will only appeal to Asians or foreign people who like Asian music or culture. Living here in the US, I could never imagine a song like We are the Queens or Warning playing on the radio. But Utada's Michi for example I think people outside of Japan could instantly get into it.
As others have mentioned, Utada is more private and low key. She controls what people know about her and any news about upcoming releases. I also think that Utada's personality is just likable. She feels like one of us who just happens to be very gifted musically and well known. To me, Ayu used to be like that but I feel she gave in to pressure from fans and avex to be glamorous and always outdo herself. Giving people who don't know her well an image of a diva or "queen". I think Japanese people are hypocritical with pop stars sometimes, they love cute stylish idols with fancy clothes and music videos but also want them to be very humble and modest.
There are just too many things to sum up in one post on this subject.

I know that I certainly love both very much. But I just have a deeper connection to Ayu. She just resonates with me in a special way that only a handful of artists do. I love Utada for her wonderful creativity and interesting voice. She is a major artist in my life. But I don't feel like I have a relationship with her because of her low key nature. She is still a little mysterious to me even after all these years. It keeps me at a distance, I really only know Utada through her music and the handful of interviews on the net.