the reason why的完整用法是“ the reason why（原因状语从句句子）......is that（表语从句）...”

所以你这个句子有错误，应该是：the reason why we call it indirect is that the term“direct translation”is not

suitable for describing....

这里的why其实引导的是一个原因状语从句，所以是完整的句子！相当于the reason for which,这里的for which用why代替，即“介词+which”可用副词代替的现象