Because Christ offered it to the Disciples. Matther 26: 27
Then he took a cup, and when he had given thanks, he gave it to them, saying, “Drink from it, all of you. 28 This is my blood of the covenant, which is poured out for many for the forgiveness of sins.
“Since Christ is sacramentally present under each of the species, communion under the species of bread alone makes it possible to receive all the fruit of Eucharistic grace. For pastoral reasons this manner of receiving communion has been legitimately established as the most common form in the Latin rite. But “the sign of communion is more complete when given under both kinds, since in that form the sign of the Eucharistic meal appears more clearly.” 225 This is the usual form of receiving communion in the Eastern rites.”
While the sacrifice of Christ is now offered in an unbloody manner (Christ does not die again), the double consecration signifies His death. The bread is consecrated first and then the wine to signify the blood being separated from the body (death). When they are mixed together again, it signifies the resurrection.
DISCLAIMER: Catholic Answers has turned over the archive to Catholic-Questions.org and no longer owns, manages, or moderates the forums. For additional apologetics resources please visit www.catholic.com.