Why is the prefrontal cortex called such?
The prefrontal cortex (PFC) is a part of the frontal lobe (Fig. 1).

Lobe means


  [A part] demarcated from the rest of the organ by a fissure (crack), sulcus (groove), connective tissue or simply by its shape. For example, there are the frontal, parietal, temporal, and occipital lobes of the brain.


Cortex means 


  The outer or superficial part of an organ or bodily structure (such as the kidney, adrenal gland, or cerebellum or a bone); especially: cerebral cortex 


The frontal lobe is called as such, because... it's in the front of the brain.

The prefrontal cortex is called that way because it forms the frontmost part of the frontal cortex. Prefrontal means


  anterior to or involving the anterior part of a frontal structure


Anterior meaning


  situated before or toward the front


Indeed Siddiqui et al (2008) mention:


  The anterior part of the frontal lobe referred in the literature as ‘pre’-frontal lobe has been simultaneously referred to as ‘frontal granular cortex’ and ‘frontal association cortex.’ The anterior most portion of the frontal lobe is occupied by the PFC...


Reference
- Siddiqui et al., Indian J Psychiatry (2008); 50(3): 202–8


Fig. 1. The frontal lobe includes the prefrontal cortex (PFC). source: Socratic