Because what Christ spoke of was not literal. God forbids eating human flesh. Period. Christ spoke of a symbolic ingestion of His words so that they became an active part of the believer.
How could eating and drinking of He who Is Uttermost Reality break a dietary code?
Symbolism. Unless you honestly believe that eating a steak will make you a cow. We are to digest Christ's words and live out our lives accordingly. Becoming Christ-like in our thoughts and deeds.
Christ, our Passover Lamb, has been sacrificed for us
The Jews were commanded to eat the Passover Lamb. How then could eating the Passover Lamb be against the law?
When we eat Christ we fulfill the Mosaic Law.
Indeed, we fulfill it more than even Solomon or David - or Moses! - ever had a chance to during their lifetimes. We do not break it, and Christ is not asking us to.
The cannibalsim argument proceeds from a misunderstanding of what substance is. What a cannibal eats is the accidents of a body, not its substance.