A parable forces the reader to hold it lightly since it's so ethereal and abstract.

Which helps people apply it to more situations, since it is a rough parallel to a lot of things and not a direct parallel to any one thing.

A concrete story is easier for people to grab onto (it's more obviously useful in some context), but harder to apply to parallel situations.

A parable grabs the reader not because it is concretely helpful, but because it is enjoyable aesthetically on its own.