The reason why [Eucharistic Prayer No. II] has its own preface lies in its origin. This prayer is an adaptation of the anaphora of St. Hippolytus of Rome (martyred A.D. 235). This is the oldest extant text of a developed Eucharistic prayer, so old in fact that it predates the introduction of the Sanctus to the liturgy.Since the Sanctus is now considered as essential to the literary structure of the Eucharistic Prayer, it was decided to adapt the first, and theologically richest, part of this ancient prayer and transform it into a preface.
Thanks for that information. I had no idea! Learn something everyday! LOL!