The rationale, such as it is, is similar to that offered my other fundamentalist Protestant groups. To over-simplify: The councils that established the works of the NT did not actually DO anything at all. They merely accepted what the church (defined as the loose group of people calling themselves Christians) always understood to be the inspired word of God. Basically, everybody already knew the four gospels were inspired and the Cathoics simply accepted what everybody already knew about which works were inspired or not.
Of course, this is simply wrong. There almost certainly was a core set of writings that most churches read as part of the liturgy. However, there was some regional variation: lots of places read from the Shepard of Hermas on a regular basis, for example. There were considerable disagreements about the book of Revelation that have not been completely satisfied even today. In some places, gnostic or otherwise exotic texts were used: Protoevangelium of James or Acts of Peter or various gnostic works.