Why is it the case that contractual liability is usually defined by basis of foreseeability rather than negligence (like in tort law)? I am looking a good explanation for that from economics. I understand that contract breach can be seen itself as negligent act. I also understand the economic reasons behind foreseeability doctrine (or Hadley-Baxendale).
Why is it the case that contractual liability is usually defined by basis of foreseeability rather than negligence (like in tort law)?
I find this statement confusing; what liability?
Your liability under a contract is to do (or refrain from doing) those things that you contracted to do. Obviously, these have to be foreseeable (at least in a general sense) or you can't form a contract around them.
Your liability with respect to negligence is not to negligently breach any duty of care you have to people to whom you have a duty of care.
If you are instead talking about quantifying damages arising from a breach of contract or the tort of negligence then the foreseeability of those damages arising from the breach or the negligence is explicit in both doctrines. Unforeseeable damage is not recoverable under contract (see Victoria Laundry (Windsor) Ltd v Newman Industries Ltd ) or negligence (see Overseas Tankship (UK) Ltd v Morts Dock & Engineering Co Ltd ).
Edit in light of OP clarification
Breach of Contract
You have to prove there was a breach. The cause of that breach does not have to be negligent.
You have to prove there was negligence. See Business sending personal info to a random email address for how to do this.