[Q] Why do the very similar galaxy variants have incompatible roms?

[invinciblegod]

I have a Samsung Vibrant and was wondering why the stock roms for the galaxy S don't work with it. The hardware is nearly identical save for the search button and the frequency. I can understand if things like the search button stop working and the gsm radio doesn't work because of the slight difference, but why does many of the other things also break like GPS? The hardware is so identical people put the galaxy S FFC into the vibrant! I was just wondering what exactly causes so much problems. Thanks.