怎么看出是不是定语从句

Davidson’s article is one of a number of pieces that have recently appeared making the point that the reason we have such stubbornly high unemployment and declining middle-class incomes today is largely becouse of the big drop in demand of the Great Recession。

这段话中为什么we have such stubbornly high unemployment and declining middle-class incomes today是定语从句，定语从句后面不应该是不完整的么？感觉更像同位语从句啊？
而且说是之前省略了that，that在定语从句中不是只能做关系代词么？可是这个句子也不缺成分啊

Davidson’s article is one of a number of pieces that have recently appeared making the point that the reason we have such stubbornly high unemployment and declining middle-class incomes today is largely because of the big drop in demand of the Great Recession。
把它理解成the reason why we have such stubbornly high unemployment and declining middle-class incomes更恰当，这是why引导的定语从句，从句是完整的，因为why是副词状语（why可以省略的）。

The reason (why) he came late was that he got up too late.
The reason （that） he gave us was not true.这个是引导词作定语从句宾语。

american english and british english differ 这是完整句子，不是定语从句；定语从句必须要由关系代词或关系副词引导。

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