They use wine and unleavened bread. Here is the reasoning:
In the New Testament, leaven (yeast) was considered to be a symbol of evil or falsehood.
"How is it that you fail to understand that I did not speak about bread? Beware of the leaven of the Pharisees and Sadducees.” Then they understood that he did not tell them to beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees. (Matt 16:11,12)
So we use unleavened bread to symbolize the Bread of Life (Jesus) in whom is no evil or falsity.
Similarly we use unleavened wine. Wine symbolizes the blood of Christ which is pure and gives life. In the preparation of wine, all yeast settles to the bottom as dregs, and the pure unleavened wine is skimmed off the top. Grape juice contains yeast cells that has fallen into it from the air.
I must point out, that we don’t consider the communion to be only symbolic, but also to be a sacrament (a means of grace).