Maybe because there isn't much need and/or not much money to be made by this ?

Your picture isn't much different from this one :



This setup is easy, simple, light and works well. Both accessories are standard and available for about 5 euro each. So why bother ?

I'd like to see Nokia-specific accessories for other needs such as keyboards, stands and cases, but the power injector is so easy... Furthermore, as long as Nokia doesn't officially support powered USB host on the tablet and there is a risk of voiding warranties by doing this, I very much doubt any third party is going to market something for this very purpose.