Hi Vishnu,
What you are seeing is normal. Due to capacity limitations they will frequently limit zone redundancy and other features to certain subscriptions based on criteria.
If you need access to zone redundancy in East US you can request access via quota support request. It is not guaranteed they will approve your request. I can post detailed instructions for creating support request if you are interested.
Please note the above [limiting feature availability] is true for other Azure services as well, not just Azure Database for PostgreSQL.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP