Apple cannot force a bank to approve the card to add to your Wallet. The bank has to send your device the Unique Token that is used for Apple Pay purchases and that is what the bank is not doing and why you are receiving the message that the card cannot be added.
First tier bank support is not likely to know what the problem is and what they will check is if there are any limits on using the physical card or if there are fraud alerts on your account. That is not the problem and since they don't know the answer they told you to contact Apple, what you will have to do is escalate the issue with your bank where you would be able to speak to their IT support that would have the relevant information. They can review the logs on their servers to see where you have requested the card to be added.
Now banks will not always tell you the reason why they are not allowing the card to be added for mobile use and may not even have the ability to override the algorithms they use for card acceptance on a device. Apple does send the bank your information and device characteristics that the banks use to make that determination. The bank will not tell Apple why they are not adding a card or sending the device the Unique Token needed for Apple Pay use.
Banks do use your Account information and the Device Characteristics to determine if they are going to allow a card to be added, for example:
- Is this a New Apple Account
- Is this a New device
- Has other cards been added to the device from a different user
- Has the user added the same card to other devices
- Has there been multiple failures when attempting to add a card on this device
- Does the region of the user match the region used by the card
This is just a few of the criteria used by banks to determine a fraud risk and if they want to allow a card to be added to this device. In any case it is the bank that will know the reason, it is just a question if they will tell you what it is or what you can do to resolve the problem. Due to the Bank Secrecy Act there is some information that a bank cannot disclose to you even if they knew the reason without facing heavy fines or possible imprisonment. Due to that, usually the information is not even disclosed to the employees where a mistake could be made and the bank held responsible. That is where you get the typical response to go ask someone else, or in this case it appears to be Apple.