I think it was just the simple matter of thinking that Cartridges meant an 8 bit system (most or all 8 bit systems had Carts and 16 bit computers didn't. Just as the same mentality as floppy drives (IBM -PC exception)- 8 Bit systems had 8inch or 5 1/2 floppies, where as most 16 bit systems had 3 1/4. I don't think there were true technical reasons other than cost involved, just the mindset of the day.
Remember, most early 8bit systems were not even made with floppy disk or hard drives in mind, they were made with the assumption that Tape drives and Carts were going to be the most convenient way of inputting and storing data, because in the late '70's early eighty's, floppy drives cost way more cash than the systems themselves (see ad below). I know with the TRS-80 Color computer, the floppy controller was actually on cartridge, so the floppy drive was super expensive. With Commodore, it actually all started (back with the pet) with a floppy drive in mind, and that was carried forward on to the VIC and C64.
Also, the simple fact that everyone used Commodore (MOS) ROMS for the Cartridges in the early days, Could have lead Commodore to promote the use of Cartridges in all the early systems, but by 1985, the cost of putting software on ROMS made no sense any more due to the fact that the floppy drive had become so affordable, that it was internal or included with all computers