Because It is entirely separate from the reality your plot hole is made to devour.

Subplot energy is a byproduct of plot energy however, once created it has the capabilities of becoming independent. For lack of better words it creates an existance seprate from reality itself. A sub reality if you will. Since the plot whole only devours matter and energy from the reality that plot energy exists in, it cannot devour subplot energy, for it exists in a completely different reality. and ISE Field is what is created when Subplot energy is harnessed and pulled adjacent to what would be the plot reality, this field can be entered and exited freely by anything with a physical make up, however plot energy and any other form of energy other than sub plot energy cannot pass through the field. Basically our characters are not within the same reality that the plot hole exists in, they are adjacent too it.