The big advantage of cartridges is that they are more rugged, easier to transport and harder to duplicate.
IANAL, but I'm pretty sure you are incorrect about the IP protection available to "mask-works". The mask-work protection laws protect the mask itself from being copied or being used w/o permission to create computer chips. The theory of mask-work protection is that the actual mask sets used to fabricate integrated circuits weren't sufficiently protected by copyright or patents. As a rule, copyrights do not offer protection against most utilitarian aspect of industrial design objects (e.g., so you can't get around the fact you can't patent a fork by allowing it to be copyrighted). Since a mask is pretty much all utilitarian, and although you can patent the chip that it makes, how do your protect the mask itself from being duplicated or used w/o permission? That's where these smask-works protection laws came from.
It is unclear that mask-work protection laws extend to actually protect the ROM contents represented by the mask (although in their "dumped" form, the software and game artwork is most certainly protect-able by copyright). In fact many statutory overviews I've read about mask-work protection seem to indicate that protection is only given to masks used to make a specific topological pattern of circuits on an integrated circuit, but not protect a circuit that is potentially functionally the same but a different pattern (e.g., a ROM that is topologically different, but functions the same). You could of course attempt to protect the functionality of your IC by a patent, but it would not be protected by mask-work protection laws. It might be hard to assert a new patent on a ROM device, except for potentially a novel copy protection scheme, so I'm not so sure how mask-work protection helps from a legal point of view over simple copyright.