Noa will say she doesn't know how people are specifically chosen, but she says she became a Guardian only after accepting the mandate Creation gave her: to use her soul to fight the darkness, to strive to preserve life.

Constantine will chime in to add that the strength of a Harbinger or Guardian's powers are fundamentally drawn from the same source -- their Spark, a magical focus manifested from the soul. As Void strives to turn life to entropy, it's natural that it would choose a vessel that has the power and potential to do so.