While stories centered around Asian women that have romance don't have to include Asian guys, it still strikes me as weird that this story doesn't have any. I believe this story is not an autobiography, but based on life experiences.
That means the story is fictional. So the way the five boys she crushes on (four white, one black) were written was a deliberate choice. However, this book wasn't written in a vacuum. I'm guessing that the author, Jenny Han, is aware of Asian American issues, including those of men.
While she is free to write the book however she wants (I mean, it's HER book), unfortunately this choice has certain implications, whether she likes it or not. Which is why you see comments like "why doesn't she crush on an Asian guy", and other such criticisms.
And for someone on the outside who isn't aware of Asian American issues, watching this might subconsciously reinforce stereotypes & ideas like "Asian girls don't like Asian guys" and stuff like that. But I believe the root cause of this is that there is so little AsianAm representation. I don't think it's fair for Jenny Han's story to bear the shoulder of all Asian representation, but given how little we do have, it kind of ends up like that.
However, in general, if we had more movies and media (mainstream or indie) depicting couples of all kinds of ethnicities together, this wouldn't be as much of an issue. Unfortunately, we're not there yet.