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Short Answer: Not just ROM code, but as well I/O space is a premium and unlike ROM size it can't be increased. There are only 7 slots and only 7 devices can be present (*1) and accessed (*2). When Apple added (intended to add) AppleTalk with ROM 0, Slot 7 was assigned. With ROM 3 this was dropped allowing the use of Slot 7 for the mouse interface. Either ...

Whilst this is not a definitive answer, by looking at Apple IIc ROM Version1, which more fully lists the features of each ROM version, it seems that the disk support was actually improved, whilst, unfortunately, removing the ability to easily boot from the second drive (although the second drive could still be accessed post-boot). One can only surmise that ...