That is the reason that he did not call.
My question is: what is the antecedent of that in the sentence?
What is the noun clause hinges on?
As is evident, this independant clasue does not rely on a noun.
We call this use of "that" a restrictive clause. Restrictive causes do infact often rely on nouns, but not all the time. Here, "that" is simply being used to single out a phenomenon, rather than a noun.