Why does the Catholic Church give Communion under one kind only?

The Catholic Answers

For many grave reasons. This custom inculcates in a practical way that Christ is completely present under either kind. It excludes the heretical doctrine that it is absolutely necessary for Communion to partake of the chalice. It removes the danger of irreverence to the Precious Blood by upsetting or spilling it. It spares the recipients the danger of infection by their drinking from the same chalice. It enables a Priest to celebrate Mass and distribute Communion without keeping the congregation an undue length of time, a reason which has particular force in the Catholic Church where hundreds go to Communion at early Masses. It secures uniformity of practice throughout the Church, for whilst flour is easily obtained for the purposes of bread, and easily kept, wine cannot be secured in sufficient quantity in many countries, above all in foreign missions.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty