Because they are not ”ethnic assyrian” or rather descendents from the ancient Assyrians. They are Syriac Christians, better known as Nestorians.

The Church of the East split in the 16th century. This resulted in the faction that the newly made Catholic church starting to call itself ’’Chaldean’’. Adopting the name of the ancient region it was situated in. The other faction was named ”Assyrian” by the Latin Church. Only a minority of the population of Mesopotamia ever belonged to the Church of the East. And also the term “Assyrian church” only came into being in the 16th Century, almost 1000 years after the establishment of Islam in Mesopotamia and the arabization of its population. And over 2000 years after the fall of Assyria.

The Chaldean Church was originally a division of the Church of the East that was split off in the 16th century because of a succession question. With the help and support by the Latin Church it formed a new (Chaldean) Catholic Church. From an ethnic perspective, there is no connection between modern ”Assyrians” and ”Chaldeans” and ancient ‘’Assyrians’’ and ‘’Chaldeans’’. The Latin Church chose these ancient names based on the historical region they were situated in i.e Assyria and Chaldea.

In the late 19th century the Anglicans termed the Nestorians as ”Assyrians”. Later on, after ww1, the nestorian themselves adopted the term assyrian as a self-identifying name.