The FHS explains
Historically there have been a number of other different places used to mount removable media such as
/mnt/cdrom. Placing the mount points for all removable media directly in the root directory would potentially result in a large number of extra directories in
/. Although the use of subdirectories in
/mnt as a mount point has recently been common, it conflicts with a much older tradition of using
/mnt directly as a temporary mount point.
The last sentence explains part of the problem:
/mnt is traditionally not a directory containing mount points, it’s the mount point itself (e.g.
mount /dev/sdc1 /mnt). Of course nowadays both
/media are typically directories containing mount points, which avoids the issue, but it doesn’t have to be the case.
Another issue is that
/mnt is explicitly the system administrator’s domain, which means that programs which want to mount file systems themselves can’t expect to be able to use it. That’s how I tend to think of
/media is where removable devices are mounted by desktop environments etc., whereas
/mnt is where I (as the system administrator) mount file systems when I need to. I never touch