Why my WhatsApp data doesn’t transfer to my new phone?

If you are using the same SIM (Phone Number), then log into WhatsApp with the same Phone number that was used to backup. If it prompts you to update chat history, then you use it or else check the Phone Number. WhatsApp chat history can be deleted via WhatsApp only.

Even if you erase your iPhone the chat history remains on iCloud Drive and is accessible only through WhatsApp.

WhatsApp Backup is taken on iCloud and while restoring, it verifies your Phone number.

• The phone number used for backing up and restoring must be the same. You can't restore the chat history from another WhatsApp account.
• Because backups are tied to individual phone numbers, it is possible to have backups for multiple WhatsApp accounts stored in the same iCloud account.

Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp

WhatsApp data is not part of an iPhone Backup either on iCloud or iTunes.

WhatsApp stores its chat backup through its own built-in App Settings on iCloud. See the pic below

If you had not enabled that then I am sorry you need to contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp

• Have you taken chat backup on iCloud through the app?
• Are you using a different SIM for the new iPhone? Then you can't.