Rental feasibility, Cart vs 3.5" floppy I doubt you would snap a 3.5" disk in half by accident, and it's not going to get damaged by accident.
Ridiculous. All it takes is a bit of weight in the wrong spot on a floppy disk and you can damage the moving parts (you know, like the little metal door?) Also, heat/liquid/dirt are much more likely to damage a disk than a cartridge, the cartridge will generally dry out and work fine if it gets wet. Floppies are inherently more fragile than cartridges, and rental items generally see abuse, end of story.
Write protection is good enough protection against accidental transmission of a virus, but some nasty renter might think it's fun to stick a piece of tape over that hole and put a virus on the disk just for kicks.
edit: not to say the media is the biggest reason games weren't rented out on a large scale. Consoles just sold way more units, creating a bigger market for rentals.