There was an extended period of time in the 80s when the cost of ICs were extremely expensive and were eating into profit margins. At the time, there were not a lot of different mappers for the NES, so Nintendo decided that an add-on system, the FDS, would be an affordable (for them and the consumer) way to get games to folks. They were right. The FDS even had extra sound, you could save to disk (this was way before battery memory, and passwords were invented for Kid Icarus and Metroid, iirc, which were both FDS disks, originally), you could recycle the disks and get new games on them cheaply in shops (NOJ had iTunes beat by nearly 20 years!). Not long after the FDS was released, maybe a year or so, maybe a bit more or less, ICs came down in price, 128kb was no longer enough memory (the capacity of an FDS disk), and many newer, better mappers were available for the Famicom. So, the FDS died, and cartridges continued to thrive. Along the way, many FDS-only games got converted to carts.

Thus endeth the history lesson.

-Rob