The address map for the Classic puts ROM at the 4mb mark, so it's kinda stuck to 4mb. All the 68000 macs use this address arrangement, with the exception of the Portable. The Portable is the odd one here, where it maps ROM at the 9mb mark.

In the 68000 machines you can't really rearrange addresses since there's no MMU. You can't remap the hardware registers, so things are kind of stuck.