Unfortunately KDP refuses to answer clearly and directly why they don't support Persian language in their services. Here's their reply:

Hello Mayhar,

Greetings from KDP.

We're unable to publish books in Persian on KDP.  We're continually working to expand our publishing tools to include the number of languages available to our worldwide customers. Please check our forums periodically for updates on newly supported languages:
https://www.kdpcommunity.comI apologize for any inconvenience this has caused. We appreciate your understanding.