why does it say erase pending when I tried to erase my lost iPad

To clarify, removing the lost device from your account will in no way improve the security or protection of any data locally stored on your lost device - but will inhibit ongoing visibility of current device status. Removing the device from your account will make absolutely no immediate difference to the status of your lost iPad and its data.

In more detail, all data locally stored on your iPad is protected by an encrypted file-system. This data cannot be accessed without knowing the device Passcode; your device Passcode unlocks the Secure Enclave (i.e., the iPad security-chip) within which the encryption keys are securely stored. Without the Passcode, the encryption keys and protected data cannot be accessed. If you iPad is found - and incorrect Passcodes are repeatedly entered - your iPad will automatically disable, rendering all locally stored data permanently inaccessible.

If the iPad is found and turned-on - and obtains a network connection - the currently “pending” instruction to erase the iPad will be delivered, causing a crypto-erase of the iPad. Again, your data will not be accessible.

You can delete the device if you should choose to do so; but doing so offers no benefit to your data security. If you are not eventually reunited with your lost device, you might delete then device. If deletion makes you feel a little better (“out of sight, out of mind”), then you might derive benefit in early deletion of the from your account.

This message indicates that Apple’s back-end servers are waiting to deliver the remote-erase command to your lost iPad.

It follows that your iPad cannot execute the erase command until such time as the command reaches the device - and the command cannot reach the device until it has been switched-on and has established a network connection via a WiFi or Cellular connection.

When/if the command reaches your iPad - and your iPad acknowledges/executes the command - the pending state that you see will change.

If you remove the device from your account, you’ll lose (a) all visibility of the device - and (b) permanently lose all ability to manage it.