Note: I'm not autistic enough to anethema anyone over this subject. Just curious about the traditions that could explain it. May all unity be restored.
I side with Orthodox over Catholic in almost all debates, but I have to side with Catholics on this one.
Whether you go by the Synoptic or John's date for the Last Supper, one who kept the Jewish law in its entirety would have no leaven in the house at this time of year. Removing all leaven is done before Passover and, oddly enough, it represents removing sin from your house.
I doubt the Lord would intend for that of which He said, "This is my flesh" to contain the chamutz that represents sin.
Of course Communion, despite similarities, is not a Passover. Otherwise you would eat reclining and there would be hand washings.
Am I being ridiculous and overthinking this? Is there a good Orthodox explanation of using leavened bread?