The theme being hit songs there would be nothing Ayu could sing that wouldn't be at least 5 or even 10 years old. She had her fair share of songs big among her fandom and even some that got notorious over the years because they were performed at concerts, but she didn't had a general public hit in a while.
Unlike Hikki who had this tie-in from a movie that "earned Japan's second-highest weekend box office of 2012 with 1,131,004,600 yen (US$13,913,200).[20] The film subsequently grossed the equivalent of over US $60 million at box office."
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In other words, unlike Ayu, Hikki had a 4 years old hit to sing that happens to be part of her next album.