One reason that companies don't succeed as often as they should is that ....

有些公司不像以往那样常常成功的原因之一是.......。

that companies don't succeed as often as they should ——定语从句。

reason 的定语从句，原则上用why 引导，有时可以用that，也可以省略。以下说法都正确：

One reason why companies don't succeed as often as they should is that ....

One reason that companies don't succeed as often as they should is that ....

One reason companies don't succeed as often as they should is that ....