Hand drawn pixel graphics heavily relied on the hardware's color palette (and monitor properties) for dithering and 'lighting' tricks, and especially the C64 color palette is quite 'exotic' and didn't have overlap with other home computers of the time. You need to consider that essentially each pixel was carefully placed by hand to 'enhance' the limitations of the builtin palette through color bleeding with the neighoring pixels on CRT monitors.

Automatic conversion of images between different hardware platforms usually stood out as looking quite poor and a sign of a 'sloppy port'.