Why was my fantasy baseball trade vetoed?

Johan Santana is a fantastic player. C.C. Sabathia was a god last year. He had 10 complete games in only a few months with the Brewers. His ERA was under 1.5. Yes Sanatana is great, but CC Sabathia is better on his own. THrow in Prince Fielder who is a lock for at least 30 HR and 100 RBI, potentially 50 and 135; It's definitely a justifiable veto.