It seems a lot of people have wanted JiffyDOS on a cartridge to save from having to open their machine. While changing a chip one time may not seem too difficult, there were plenty of 64s (such as 64C) that did not have socketted Kernal ROM. What would be easier than plugging a cart in? Of course, this project would not be limited to JiffyDOS. A lot of people like to roll their own Kernal.
Check
Commodore Bounty for details.