>>280108093
Because she is a better character than them.
None of those characters exposed the reader to their inner thoughts and feelings, so they're left a mystery in that regard. Most of them are just one-note characters that Fujimoto used to crudely push the plot forward, and admittedly himself felt no emotional connection to any of them.
With Asa, on the other hand, the reader gets to experience her full range of emotions, see her go through trauma, and come to understand her ways of thinking about the world.
Every other character we only get to witness through Denji's eyes.