[jltsiren]

The short version is that the internal politics of the Mongol Empire shuffled the religious map of Asia.

Christianity gradually spread further into Asia, but the rise of Islam severed the contact between the Church of the East and Western Christianity. Mongols had their own shamanistic religion, but many of their tribes had also become Christian, Muslim, or Buddhist. The Mongol Empire tolerated most religions, and individual leaders often favored one religion or another. When the empire fragmented, three of the major khanates chose Islam and the fourth chose Buddhism, while Christianity largely faded away in the Asian parts of the empire.