With "reported speech" the important thing is to explain the information, not the words, to your listener. The information needs to be true and make sense to your listener now. The OP's sentence is slightly complicated by the fact that it uses a question. Let's look at two declarative sentences:
Consider the following direct speech:
- It is against the law to steal.
Now, if stealing is against the law now then we can use either of the following sentences to report this:
Mary was told that it was against the law to steal.
Mary was told that it is against the law to steal.
This is because Mary was told that generally stealing is not allowed. This situation was true when Mary was told it, and so sentence (1) is correct. If Mary was told that it is generally against the law to steal, then sentence (2) is correct as well. The information Mary was giving was not intended to just be true about that particular time.
Which choice we use though, may be important. If we are trying to show that something is still true now, for example, we will probably use sentence (2). If we are trying to describe a story in the past or a situation in the past we will use sentence (1). Here is an example where it might be important to use sentence (2).
Suppose you want to tell somebody that you have a particular policy about something, and that you have checked this with your boss. Because we are interested in what the situation is now we will expect you to say:
- The Director said that this is our policy.
Now if you say:
- The Director said that this was our policy.
Then this might sound as if your policy has changed now, but it used to be your policy. This implication will be stronger because we expect the sentence to say is our policy, so we might think there is a special reason for you to change the tense here.
Of course both sentences are true and both sentences are grammatical, but in this situation, it might be better to say it is our policy, just because the listener will not be able to misunderstand the sentence.
The Original Poster's sentence:
This situation is the same as the ones above. The speaker was asking about why people cry in general. We can portray it as a question about the general situation at the time of speaking or as a general situation from the current perspective. Remember, it is the information that is important, not the words. We can use either of the following:
- She asked why people cry.
- She asked why people cried.