[bydo]

Back in the day nearly every "port" was a full reimplementation, from the code to custom graphics and sound. Such are the perils of writing to bare metal.

[mattl]

There are lots of ports from the 8-bit home computer era which don't have this problem. See for example, the number of ZX Spectrum ports to the Amstrad CPC machines.

[Narishma]

Yes, and they usually were terrible ports. They ran poorly and didn't take advantage of the CPC's better hardware.