Why did distinct Mongolian languages begin to develop during the Middle Mongolian period?

During the Middle Mongolian period (13th–16th centuries), distinct Mongolian languages began to develop primarily as a result of geographic separation and isolation. Following the expansion of the Mongolian language from its original homeland at the end of the Ancient Mongolian period, the subsequent recession of the Mongol conquests left various groups isolated from the main body of speakers.1

These isolated populations, located in regions such as Afghanistan (Moghol), the east (Daur), and the south (Monguor, Bao’an, and Santa), were cut off from the central dialects. Consequently, these outlying languages diverged from the main group, preserving archaic features—such as specific consonant sounds and unassimilated vowel sequences—that were eventually lost or merged in other Mongolian languages.1