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。
The reason is that she felt limited by her fracture, which made it difficult to continue her activity.
Because she felt limited by her fracture (or injury). 『なぜなら彼女は骨折によって限界を感じたからです。』 とシンプルに表現できます。
「活動を続けていくことに限界を感じた」というニュアンスをしっかり伝えるには、以下のようなフレーズが良いでしょう。
The reason is that she felt limited by her fracture, which made it difficult to continue her activity. 『理由は、彼女が骨折によって限界を感じ、それが活動の継続を難しくしたからです。』
アメリカ合衆国 
