Good explanation of why it's optional in this case, although i'm not convinced that reason is the only reasonable antecedent of why.
For example, the explanation why is a common usage,.
I don't know why, but it seems to me bob would sound a bit strange if he said, why is it that you have to get going? eliminating 'that' before 'bob' would seem to be more in context.
Why should the first person pronoun 'i' always be capitalized?
I realize that at one time a lot of nouns in english were capitalized, but i can't understand the pattern of those.
I have flunked the exam, that is why i am attending coaching classes.
I have flunked the exam, which is why i am attending coaching classes.
Is there any change in the meaning of sentence.