I would guess mostly because python interpretes are just about everywhere.
Also the binaries compiled with nuitka end up being much bigger in size. A simple script of a few kb can and up in the hundreds of mb when you start compiling the dependencies, so it's not a perfect solution.
I went with chafa as it's terminal agnostic and supports various modes.
Then again, I'm not really sure a tui frontend needs high quality image rendering. Earlier I even considered going completely 1bit braille or just ASCII just so that the image doesn't take all of the focus at the expense of the post body.
As mentioned by another commenter, I believe opening the full image in an external viewer is a much better solution, not to mention easier to implement.