Your category layout is quite different as it uses the subforum-list function and layout in bbPress, which you would need to filter to include the description like you want.
But since this seems similar to another file that I have that can create this layout, take a shot at using it instead. Take this loop-forums.php file and put it into your theme and see if it will make the layout how you want it with the forum description.
https://github.com/robkk/bbPress-Hierarchical-Forum-and-Category-Layout