Why can’t I get text messages on my iPad that are on my iPhone

SMS text message (not iMessages) are send to a phone number, not to an Apple ID. Your iPad doesn’t do cellular text messages, nor phone calls. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services.

With Text Message Forwarding, for new conversations (it doesn’t apply to past messages from before setting up syncing), other Apple devices with your iCloud account can virtualize the sharing among your devices through Continuity, but still the phone will be used for actual sending and receiving.

If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from your iPad you must explicitly enable Text Message Forwarding for your iPad on your iPhone...

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON