It's because Jesus commanded (instituted) it so...
Mark 14:22-25 NKJV
 And as they were eating, Jesus took bread, blessed and broke it, and gave it to them and said, "Take, eat; this is My body."  Then He took the cup, and when He had given thanks He gave it to them, and they all drank from it.  And He said to them, "This is My blood of the new covenant, which is shed for many.  Assuredly, I say to you, I will no longer drink of the fruit of the vine until that day when I drink it new in the kingdom of God."
1 Corinthians 11:23-26 NKJV
 For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread;  and when He had given thanks, He broke it and said, "Take, eat; this is My body which is broken for you; do this in remembrance of Me."  In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me."  For as often as you eat this bread and drink this cup, you proclaim the Lord's death till He comes.