Eu acho que está lá desde os tempos antigos e, provavelmente, por vários motivos de compatibilidade.
Algumas explicações do subcapítulo de CD-ROMs do The Linux Cookbook seguem aqui:
Like the
/floppy
directory, the use of the/cdrom
directory is a standard practice and convenient, but not necessary -- you can mount disks in whatever empty directory you like. (You could even, for example, mount discs from the CD-ROM drive to/floppy
and mount floppy disks to/cdrom
, but why would anyone do that!)
Além disso, encontrei este post no Ask Ubuntu que tem a resposta definitiva para a sua pergunta:
While
/mnt/
and/media/
are common places to mount devices, the device location can be just about anywhere./cdrom/
was probably chosen for either brevity or legacy support.