Why can many Tajik speakers understand Persian?

Many Tajik speakers can understand Persian because Tajik is a member of the southwest group of Iranian languages and is closely related to the Persian language.1

Specifically, Tajik is considered mutually intelligible with the dialects of Farsi spoken in Iran and Dari spoken in Afghanistan. While these languages share a common linguistic foundation, they are often distinguished by their writing systems: Tajik is typically written in the Cyrillic alphabet, whereas Modern Persian generally uses Arabic characters. Despite these orthographic differences and some regional variations, the close genetic relationship within the Indo-Iranian branch of the Indo-European language family ensures that speakers of these varieties can communicate effectively with one another.2134