[susan53]

An interesting one. Quirk et al (The Grammar of Contemporay English) point out that the present simple is often used with "verbs of communication" when the communication is in fact past - eg : John tells me you're going to Japan next week suggesting that the meaning is that the communication is still "in the air" and present. But this is a description, not an explanation. Logically, if we see it like this we'd expect the present perfect (expressing a past event with a present result : he told me so I now know...) John has told me that you're going...

So why do we choose the present simple? Normally it expresses unchanging facts - eg Oil floats on water. But of course that doesn't always mean that the "fact " will exist for ever. If I say This soup tastes good, the statement obviously isn't going to be true tomorrow - by which time I'll have eaten all the soup. It has to be understood as meaning "For the time that this soup exists this statement is true and will not change."

I'd suggest that something similar is happening here. You asked a question which has not yet been answered. So the question - ie your "asking" - still exists. And for all the time that it exists (ie until it's been answered) it's seen as an unchanging, permanent fact.

So :
Does the soup taste good? / Why do you ask?
The soup tastes wonderful! / I ask because I thought that....

That's as close as I can get...

Obviously, we could see it differently - as an on-going event limited in time - and therefore use the present continuous : Why are you asking? As always grammatical forms express specific meaning concepts, and it is the choice of the speaker which one they want to "mean".