They were proprietary and made by only one company. So there was a bit of a battle between Zip, SyQuest and LS120 ("SuperDisk")
What I don't get is why Magneto-Optical didn't take off. 3.5" disks that were essentially MiniDisc but not proprietary (there were drives and disks from several manufacturers), faster than Zip, and eventually got upgraded to over 1 GB capacity disks. They were huge in Japan where they became a de-facto standard, but mostly unheard of in the rest of the world.
They sorta did in the Apple world. Maybe not quite a standard, but quite common. Apple shipped several Macs with (optionally) built-in Zip drives in the late 1990s.
