People were sick of Trek at that point
Correction, people were sick of ST: Enterprise.
An academy show, assuming that it was written and cast correctly (big assumption) would be great for the youth demographic. King Daniel Into Darkness mentioned the fantasy series Smallville, which has been on the air for a freaking decade, pulling decent if not blockbuster ratings.
I've a feeling the ratings on any post Enterprise
show would make make the studio and network long for Enterprise
's numbers. IIRC, every show after TNG had ratings lower than the previous one.