Because MsDos, at least in its early versions, could not dynamically assign new drive letters to newly inserted disks. So the workaround, which is still with us years later in Windows 7, was to reserve at boot time the first two letters (A/B) for drives with removable media (i.e., floppy drives, there were no usb flash dongles in those days).
And this was perfected in Windows 95 which always had drive A:\ visible in My Computer even on computers without actual floppy drive (in that case it was named "Removable drive", had generic removable drive icon and did nothing useful).
