What exactly is wrong, why are these png format illustrations not accepted?

Let's first check your cut-out:

It's exceptionally clean, but still contains errors like these ones:

The blue points out to areas that are at the border of a transparency. So inside of the full material, you have islands of tranparent pixels. That will greatly annoy a possible buyer when they put it against 

I also think that thge cutout circle is not correct, and that the cutout should be smoother. 

Second, you have rendering errors like these ones a little bit every where:

Third, there are inconsistant sharpness levels in your file. That too will be highly annoying to a buyer, as this is visible even when the asset is not shown full size:

I didn't check the other assets, but I suppose they are all the same with more or less similar errors. 

In conclusion: assets like this are easy to model in a 3D program, and you really should avoid doing this in generative AI, because generative AI obviously still has no clue what they are rendering.

Do your illustrations have a transparent background? Because PNG is reserved for transparent backgrounds

Some inconsistencies in the drawing: