419. 「たる所以は」

「リーディングメーカーたる所以はその製品群にあります。」訳しにくい日本語表現ですが、妥当と言える翻訳案を挙げるなら、

The reason why the company has been able to maintain its position as a leading manufacturer lies in its array of products.

あるいは、語順を入れ替えて、

It is precisely because of its range of products that the company has been able to maintain its position as a leading manufacturer.

といったところでしょう。