Why is WhatsApp unavailable on iPad

Expanding upon MrHoffman's reply...

The WhatApp developers have not [yet] released an App compatible with iPad - the App being currently limited to iPhone. Whether or not a compatible App will be released at some future data is known only the the App Developer.

Whilst the App is not available for iPad - you should be able to access the service using a web browser:

https://web.whatsapp.com/

You will want to ask that of Meta, the vendor that provides and supports the WhatsApp app and the service.