Final answer:
Barbara Allan cannot forgive him because he had betrayed her.
Step-by-step explanation:
In the given text, Barbara Allan cannot forgive him because he had betrayed her. The lines "If you forgive me, Bess, it'll all come right in the end!" and "After all, I loved him. He was good to me. I can't forget that." indicate that he had done something to hurt her and she cannot overlook that.