Why Do Orthodox use Leavened Bread For Holy Communion and Catholics use Unleavened?

Following Apostolic Tradition the Orthodox Church uses only leavened bread for Holy Communion. This is affirmed by the Greek term "ἄρτος" (artos), used in Scripture to refer to everyday leavened bread of the time. The rising of bread due to leavening symbolizes Christ’s Resurrection. The yeast "adds life" to the bread, like His Resurrection brought new life to the world. Also how the yeast is worked into the flour symbolizes the divine nature of Christ permeating His humanity. 

This bead is prayerfully prepared by a devout parishioner. It is called prosphora, derived from the Greek word meaning "that which is offered.”  Before baking, it is stamped with a seal containing Greek letters IC XC NIKA ("Jesus Christ conquers”). Prosphora is made from four simple ingredients: unbleached flour, yeast, water, salt. It is prepared for offering during the Divine liturgy in a service called Proscomede, preceding the Liturgy itself.

In contrast, the Roman Catholic Church uses unleavened bread, often in the form of thin wafers. This practice is derived from a later interpretation that the Last Supper was a Passover meal, during which unleavened bread, as per Jewish tradition, would have been consumed. They also see it as a symbol of the sinless nature of Christ. The Orthodox support their view based on the Tradition of the Apostolic Church. The Orthodox Church always places an emphasis on preserving the practices called Holy Tradition of the early Church to ensure spiritual continuity without change.

The different practice in the Roman Church developed over time. They began adopting the use of unleavened bread in the 8th to 9th centuries leading to a significant difference among others between East and West. Historical evidence shows that leavened bread was originally used in all churches. By the time of the East-West Schism in 1054, this difference was a clear symbol of differences between the two Churches.

Despite this difference of leavened versus unleavened bread, the essence of the Eucharist transcends them. Both the Orthodox and Catholic Churches affirm the transformation of the bread and wine into the Body and Blood of Christ— a central tenet they share. The real presence of Christ in the Eucharist surpasses the importance of the type of bread used. Holy Communion remains a mystery of faith that surpasses full understanding, but remains a central belief in both the Orthodox and Catholic churches.

Guide for Making Prosphora

https://www.orthodoxprayer.org/Divine_Liturgy/Prosforo.html