Possibly because of the way Workbench was doing things a bit more directly? I mean, an emulator could take the two memory areas and make the OS running in emulation think it was one memory block, but who would be able to provide this feature to AmigaOS itself? There's nothing running above there, is there? That's why this would work in applications but not the OS itself, I guess...
That said, maybe it'll be possible later on :-)
