What's the main reason you wanted to try this job?
1) What made you want to do this job?
what made you~? で「何があなたをそうさせたの？」というニュアンスになり、「なぜ今の仕事をしたいと思ったの？」という風に訳すことができます。
2) What's the main reason you wanted to try this job?
「この仕事に挑戦したいと思った一番の理由はなんですか？」
What's the main reason ~？は何かの理由を聞くときに使える言い方です。
ご参考になれば幸いです！
What has been the reason why you decided to get involved with your current work?
What has been the reason that you decided to get into your current job?
What made you want to get into your current line of work?
Joさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、いくつか自然な表現を紹介します。
- What has been the reason why you decided to get involved with your current work?
- What has been the reason that you decided to get into your current job?
- What made you want to get into your current line of work?
- to get involved with___ = ～に関わる
- line of work = 業種
お好みに合わせて使い分けてみてください。
ご参考にしていただければ幸いです。
日本 
ドイツ 
