According to St Thomas Aquinas, there are three reasons for the Eucharistic fast:
1) First, as Augustine says (Resp. ad Januar., Ep. liv), "out of respect for this sacrament," so that it may enter into a mouth not yet contaminated by any food or drink.
2) Secondly, because of its signification. i.e. to give us to understand that Christ, Who is the reality of this sacrament, and His charity, ought to be first of all established in our hearts, according to Mt. 6:33: "Seek first the kingdom of God."
3) Thirdly, on account of the danger of vomiting and intemperance, which sometimes arise from over-indulging in food, as the Apostle says (1 Corinthians 11:21): "One, indeed, is hungry, and another is drunk."
Summa Theologica III.80.8.