The thing about the question Why? is that it is so easy to ask and yet so hard to answer. So use it with great care: it might make the client feel intimidated or that he/she has to overlly justify their actions.
If you need to ask why?
- do it later in the conversation once some rapport has been built
- soften it with may I ask why?
- praise the decision, then ask why it was made...."it seems a great route you have taken; what was behind your thinking?'
- suggest a possible why and ask if you are correct...." so was the reason, the fact that budget was limited?"