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,. Thus we say: You never know, which is why but you never know. That is why and goes on to explain:
There is a subtle but important difference between the use of that and which in a. I don't know why, but it seems to me that bob would sound a bit strange if he said, why is it that you have to get going? in that situation.
