Or more to the point, God.

To christians, It has always been an important belief that no being, whether corporeal or metaphysical, can be on equal ground with God. Therfore, the term "Prince," in this instance, is not meant to imply a king or queen of darkness, but is instead used as an expression secondary to "King," which is God's title. In other words, you wouldn't call Satan the "King of Darkness" because not only does it have less of a ring to it (Religion is, of course, all about the aesthetics), but you would also be implying that Satans power is on par with that of God. This would obviously be heresy, and grounds for a Burning at the Stake.

Another argument, however, is that the Prince of Darkness' father is Darkness itself. This is an idea explored in the film classic Legend directed by Ridley Scott, starring Tom Cruise, Mia Sara, and Tim Curry as the Prince of Darkness. All of which, naturally, are the subject for another writeup.

Disclaimer: All of the author's conclusions are based on supposition and conjecture, and he is not to be trusted. Elvis may or may not be God.