Why is the Prodigal Son a parable?
If you are talking about the Biblical story of the prodigal son, it is a parable because it is a story that is meant to illustrate a point. It is meant to illustrate the idea of God's mercy.
A parable is a short story that is meant to convey some moral or religious message. In this case, the message that is conveyed is that God is willing to forgive people who have acted badly. Because of this, there is always hope of salvation for people, no matter how badly they have acted in the past.
This story is not just meant to be "hey, look at this interesting thing that happened to this younger son." Instead, it is meant as an illustration of this fundamental idea about God. This makes it a parable.
The parable as a form contains many characteristics of fiction: plot, character development, description, dialogue, and a consistent point of view. The principal difference is that in the parable genre, literary characteristics are less important than theme and theology. This helps to bring the didactic lesson to the forefront and not get lost in the literary diversions of the story itself. the length of the parable is a noteworthy characteristic as the parable form can not have an indeterminate length, but is quite short in the number of verses that it generally must contain.