Supposedly, the gospels of Peter and Nicodemus would've been included in the canon except that Iranaeus insisted that, "...just as there are 4 winds, there must be 4 Gospels, for the Holy Spirit, the inspiration for all divine writing, is embodied in the wind." (from "Adversus haereses" translated by Solomon J. Schepps)
The Gospel of Thomas was rejected as heresy. The accounts of Christ's infancy were excluded because they didn't portray Him in a good light.
There is also a Gospel of the Birth of Mary, but I don't have any explanation as to why it was left out.
If I find this anywhere I'll post it.