It depends on the preceding statement. e.g. "I want to know more about the book - that's why I'm asking about it." "My question is important - that's why I'm asking it". You can save complications by saying only : "that's why I ask". This is equally good in both cases.

I would use one of these: - That is why I am asking about it. (said while you are asking) - That is why I asked about it. (said after you have asked) Of course, you can also leave off the "about it" part, since it is obvious and just say: - That is why I am asking. (said while you are asking) - That is why I asked. (said after you have asked)