Yes, you are responsible for paying the bill. Although you asked why no one wanted the dog and "was anything wrong with her" it does not sound as if you were given a guarantee of good health. In addition,while a dog that has had a bout of pancreatitis is more likely to have another bout of it, that doesn't mean the dog has "anything wrong with her". However, you can and should talk to your vet about the possibility of reducing the bill.
I am sorry for your losses...
Hard to say without more information, but you could conceivably have a defense if they sue you for nonpayment. If they knew or had reason to know that whoever adopted the dog would likely face very expensive vet bills, they had a duty to at least advise you of that. On the other hand, that is something that may be very difficult to prove. And, there is always the possibility that they honestly thought the dry eye was the only existing ailment. (Your own experience with - and faith in - this animal hospital can be your own guide in determining how much benefit of the doubt you wish to allow them.)
It may be prudent to consult with a local attorney who can discuss your situation with you and give your some guidance. Good luck...
(THESE COMMENTS ARE NOT LEGAL ADVICE. They are provided for informational purposes only. Actual legal advice can only be provided after consultation by an attorney licensed in your jurisdiction. Answering this question does not create an attorney-client relationship or otherwise require further consultation.)
I'm so sorry you lost your beloved pet.
Under general principles of law, I see nothing in your account that constitutes a defense to payment of the bill. The vet and staff told you what they knew, but they didn't willfully conceal anything about the animal so that you would adopt her, and they didn't guarantee your animal wouldn't become sick in the future. So you adopted her, you took her for treatment, the treatment turned out to be expensive, and the animal ultimately died.
You can ask them if they will accept payment of a reduced amount. They don't have to say yes, but they might.
Don't take this as legal advice as you and I don't have a confidential attorney-client relationship and I don't know all the facts and circumstances. It's just my take on the facts you present in light of general principles of law.