You are saying "thank you" to me for my reply. The verb itadaku is, as you said, a humble form, so you can only use it to talk about what you did. Obviously you don't want to thank me for what you did, you want to think me for what I did. For what I did, you need kudasaru.
Another way to look at it is that, ignoring all the politeness, kudaseru means "give" and itadaku means "take", so what you wrote means (approximately) "thank you for taking your reply" or (slightly more accurately) "thank you for me taking your reply".
I'm pretty confident I'm right on this one (this does not happen often). However, I know for sure that native speakers use itadaku when they should use kudaseru. I'm also pretty sure that this causes older native speakers to wonder what's wrong with young people these days.
![Wink :wink:](/static/images/forum_ro/smilies/icon_wink.gif)
マイケル