The reason why she retired is because she felt her fractured limited how much she could contribute.
She retired because she had a feeling she could no longer help out as much after her fracture.
ご質問ありがとうございます。
「なぜなら彼女は骨折によって活動に限界を感じたからです。」は英語で「The reason why she retired is because she felt her fractured limited how much she could contribute.」と言えます。
また、「She retired because she had a feeling she could no longer help out as much after her fracture.」にも言えます。
2つの英文は同じ内容ですが、言い方だけ違います。
どっちらでも使えば、構いません。
ご参考になれば幸いです。