Tape allows for a disconnected medium that isn't active. Moving to disk, often means that you have a process there. you could make this offsite, and offline, but then you are connecting some of the time, which is when a hacker (or more likely, insider) can attack.

If you move to tape, and physically remove tapes from machines, you have a layer of protection in the physical act.

this is less a hacker concern, and more a disgruntled employee concern. A good reason to have someone else other than the admin handle tapes. Even at a small company, I used to do the backups, but we had a secretary handle the tapes. Granted, I could have stopped backups, but we did perform a restore from tape periodically and the secretary got the tape from the offsite company, so I would have a limited window to do damage.