(i) Because preceded by not sometimes leads to ambiguity. " I did not write that letter because of what you told me " may mean either "I refrained from writing that letter because of what you told me" or "It was not because of what you told me that I wrote that letter". Avoid this ambiguity by rewriting the sentence.
(ii) To say " the reason is . . . because " is to say " the reason is " twice over, since because is a conjunction that introduces a reason. You may say " I went because I was asked to go " or " the reason why I went was that I was asked to go ", but not " the reason why I went was because I was asked to go ".
See also Not.
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