I would surmise that one could have created cartridge-like expansions for the side port. This was done for several game cartridges for the TI-99/4A (like "Miner 49er") to take advantage of the 16-bit bus available at the peripheral expansion port.
(IIRC, it was also to allow the use of larger ROM sizes, as the cartridge port was limited on ROM space in favor of the use of GROMs, specialized ROMs which used serial read sequencing rather than addressed space.)
Anyway, back to the point, I suppose the question becomes, why? Yes, ROMs were expensive then, far more than floppies. As well, I am sure people happily accepted load times for floppy-only games rather than removing their side-car expansions (even though for some floppy-only games you have to disable the expansion if it is an accelerator or has a bootable controller.) And in terms of Commodore, we already know they were cheapskates, so I seriously doubt anyone there could have justified the use of ROM cartridges where other, less expensive, media and manufacturing processes existed.
An additional concern would be the wear-and-tear on the contacts which plagued every other cartridge-based system. I shudder at the thought of an Amiga-related "blow into the cartridge" legacy, a procedure proven to be ineffective as the coincidental "fix" was actually re-seating the cartridge.
But more paramount to me would be the increased exposure to static discharge, as if we did not already have enough of that to be concerned about.
