
Because SCSI works well with removable media, and works well with large
capacity devices. The floppy interface you suggest handles the former, but
it doesn't have any hooks for dealing with the latter... you'd have to kludge
it. Plus, it's extremely low performance. AND, SCSI has gobs of room for
expansion compared with a floppy (I can just see it, let's stick a 5.25",
a 3.5", a tape drive, and a MO drive, all on the floppy interface. The
possibilities for unexpected collisions are enormous).