not disappointed with the DLC news honestly, I'm just glad to get the poses thing.
The reason is entirely technical; itch does not have any backend API or infrastructure to support DLCs and such, so the implementation would need to look entirely different... which is to say a lot higher effort.
Alternatively I could include, like, a "Complete Edition" that you unlock by paying more, via Name Your Price; but I am not sure this would be a good solution.