The attack was sophisticated, but Ledger appears to have taken almost zero precautions against hardware attacks, which made this attack simpler and cheaper to pull off. There are very few precautions that would have completely eliminated the possibility of this attack, but it would have been straightforward to make it more expensive, primarily by investing in tamper proofing and tamper evident design. There are several other comments in this thread that go into fair detail about some examples of these techniques.