The author wanted to tell a personal story influenced by his own life experiences and the people in it. He wanted the character to be an Asian teenager who succeeded his grandmother as the next GL, not just for diversity, but because the author is himself Asian whose grandmother was a strong woman that taught him important lessons in life. The fact that the book happens to have an "Asian lead" seems to be of rather secondary importance, he just wanted to tell a story that was dear to him and DC liked his pitch. That's really all there is to it. If DC wanted to use an existing GL they would've mandated so, while it seems like they just wanted a good story.