Because before that you had things like syquest drives for large portable storage, and the media was huge
This is a 1gb disk, it’s like 5 3.5” floppies stacked.
Before this things like 44-80mb disks were the same but 5.25” and required external drive housings for an entire 5.25” drive bay
And that was the most reasonable way to get large external and portable storage
Zip drives and disks and things like LS120 were nearly the size of 3.5” floppies and way cheaper, and could even be easily integrated into machines of the time. Lots of laptops from the 90’s for example had modular bays to drop in a zip or ls120 drive
Zip drives and disks were just the progression before Minidisc, and then CD became dirt cheap.