The main limitation was the speed with which the computer could process the data from the drive. To keep costs low standard floppy drives were fairly simple devices, relying on the computer to do the decoding and detect things like the start of track markers.

Later faster floppy drives became available. 2x USB floppy drives were not uncommon, and even 4x units existed. However those had their own custom controllers to handle the higher rotation speed and thus data rate.

The main limitation was the speed with which the computer could process the data from the drive. To keep costs low standard floppy drives were fairly simple devices, relying on the computer to do the decoding and detect things like the start of track markers.

Later faster floppy drives became available. 2x USB floppy drives were not uncommon, and even 4x units existed. However those had their own custom controllers to handle the higher rotation speed and thus data rate.

For example, consider the classic Intel 8272A and NEC UPD765A floppy controllers, which were very common PCs from the 80s.

Both parts operate from an 8MHz clock, relatively fast in the 1980s when CPUs were typically running at 1 to 5MHz. As speeds get above about 10MHz special consideration is needed to make the electronics work reliably, which increases cost and complexity.

Note that at 8MHz both those parts are limited to single and double density disks. They can't even handle 1.5MB high density disks at this speed. And this is dedicated controller hardware, earlier systems that used the CPU to control the drive directly in order to reduce costs were limited to single density (350kB) 3.5" disks or 5.25" disks.