I think it's mostly because you'd have to go the whole mile and redraw every single portrait, which would be quite the workload to take.
To be honest, I think this is an easy issue, even. Sure, it's somewhat of a big task, but it all boils down to money. Suppose the Dungeon does a collaborative effort on the money issue, we could get an artist to do it. No problem.
The true issue are the technical aspects. I, at least, have no idea how many tiles does each portrait occupy, nor the palettes used. One has to know how to extract the files (both portraits and palette) so the artist knows what colors and what area he's allowed to use, and has to know how to insert them back.
In the optimal case, the artist could simply draw to their heart's content (provided the portraits are somewhat proportional to the ones in the game), and someone else would reduce the drawing and the colors using the palette inside the game (I know how to do that, but I have no idea how to extract the palette).