Tapes are simple and compact. I haven't looked recently, but 40Gb tapes were available 5 years ago.

DVDs can't match the storage (haven't checked Blu-Ray, but the technologies a bit new for general use).

Removable hard disks can match or execeed the storage but come with a mass of sensitive electronics built into the media which could fail.

Hence tape is still the backup medium of choice in most situations.