For there is one God and one Mediator between God and men, the Man Christ Jesus~ 1 Timothy 2:5 (NKJV)
In order for both God and man to be equally represented, the Mediator needed to be able to relate to both parties. And only Jesus, as the God-man could do that. Only Jesus meets the requirements of a sufficient representative, or Mediator, between God and man. Why? Because He alone is both God and man. Jesus alone could mediate between the divine and the dust. Because He alone fully and perfectly represents both.