Why can't I change my PSN ID?

This has bugged me for the longest time. We all have that username that we regret using and on Playstation you have no hope if you want to change it. You can always make a new one but then all trophy information will be gone.

I know on Xbox it's possible to do (I think for a fee). I would gladly pay $5 bucks if I could change my username on Playstation. Anyone know why we can't do it and Xbox can?