I can't seem to find a decent transcript of the movie, however, didn't Pike say something about there were senior people in Starfleet who thought that Kirk had been promoted too quickly?
Pike's decision might have taken into account the position of others in Starfleet Command who also read the two conflicting reports. Pike was part of the chain of command, his decisions didn't exist in isolation.
(and why doesn't
chakoteya.net have a transcript, it's been nearly a year?)