Tape is not as sensitive to drops, bumps, or handling-cause issues that plague hard drives. They also offer a relatively low cost per gigabyte, compact size, and decently high speed. Hard drives, because they constructed of glass or ceramic disks and many movable parts, in addition to expected data degradation over time, are simply not up to the task of archival storage.
That said, most enterprises use multiple levels of backup, including both various arrays of hard disks working in combination with tape backups.
Hard disks offer fast backup and recovery as well as easy single-file restores. Tapes offer long-term archival storage, ease of off-site transfer and storage, large capacity, and small size.
Their differences do not point to a general winner overall, but each have their strengths and make them appropriate for specific purposes and strategies.
In my office we have incremental backup over WAN for easy and fast, single-file and point-in-time restores at user request, and tapes that are shipped off-site on a rotating schedule based on nightly backups.