Why did the Ethiopian Church follow the Coptic Church of Alexandria after the Council of Chalcedon?

The Ethiopian Church followed the Coptic Orthodox Church of Alexandria in rejecting the Council of Chalcedon in 451 CE because both churches adhered to the doctrine of miaphysitism.1

While the Council of Chalcedon affirmed that Jesus Christ possessed two distinct natures—human and divine—the Ethiopian and Coptic churches rejected this "dyophysitism" (two-nature doctrine). Instead, they maintained that Christ’s humanity and divinity were equally present through the mystery of the Incarnation within a single, unified nature. This belief is reflected in the name of the Ethiopian Orthodox Tewahedo Church, as tewahedo is a Ge'ez word meaning "unity".12

Because they rejected the Chalcedonian definition, these churches were labeled as "monophysite" heretics by the Roman and Greek churches, leading to their isolation as Oriental Orthodox bodies. Consequently, the Ethiopian Church maintained a close theological and administrative bond with the Coptic Church, which historically appointed the Ethiopian archbishop, or abuna, for many centuries.12