First of all, because no two substances in nature better symbolize unity than bread and wine.
As bread is made of a multiplicity of grains of wheat, and wine is made from a multiplicity of grapes, so the many who believe are one in Christ.
Second, no two substances in nature have to suffer more to become what they are than bread and wine.
Wheat has to pass through the rigors of winter, be ground beneath the Calvary of a mill, and then subjected to purging fire before it can become bread.
Grapes in their turn must be subjected to the Gethsemane of a wine press and have their life crushed for them to become wine.
Thus they symbolize the Passion and Sufferings of Christ, and the condition of Salvation, for Our Lord said unless we die to ourselves we cannot live in Him.
A third reason is that there are no two substances in nature which have more traditionally nourished man than bread and wine.
In bringing these elements to the altar, men are equivalently bringing themselves.
When bread and wine are taken or consumed, they are changed into man’s body and blood.
But when He took bread and wine, He changed them into Himself.
– Bishop Fulton Sheen