Why was Haigha a shadow?

In the last Looking Glass adaptation, Hatta and Haigha were made completely different characters than the Hatter and March Hare from the first book. At least in this one, at least Hatta is shown in prison and Haigha keeps a generally rabbit shape. But why is he a shadow? Him becoming a messenger doesn't change his composition. It's odd considering Hatta was shown to be a normal person.
This and the absence of the Lion/Unicorn scene slightly soured what i think is the best Looking Glass adaptation for me. I would at least like to know the reasons behind these odd (or lack of) occurrences in the film.