Well, from WWE's point of view:
Booker- retiring in a year or so, not worth pushing
RVD- generally given up on
Cena- they dont want to do face vs face, or turn either he or Eddie heel
Taker- proven to be a non draw as champ
Rey/Chavo- cruisers, WWE will never allow them to main event a PPV
Haas- a midcard tag team wrestler, not even remotely hoss like
So there, thats why they would go with Mordecai. Not saying its right, but thats the logic.