Comment by GTP
I'm not sure how much write-protection would be useful in practice. If you're restoring from backups after a malware infection, you wouldn't directly restore on the infected system. You would first reinstall the OS/restore some earlier snapshot and then restore the data.
The point is you may not know a system is infected. There's more use cases for storing data on tape than only making backups. We clear data off spinning disks that we aren't actively working on. When we need it again, we get it from the tape. With a hard drive, you plug it in and the compromised system can infect the hard drive without you knowing. The malware doesn't have to actively destroy things for it to be a problem, it can take action at a later date, so with hard drives they can be a problem even when you don't know you've been compromised, and even after you've fixed the compromise if you've ever plugged in that hard drive while you were compromised. There is no write-protect on a hard drive that I know of. Tapes even come in "WORM" variety which is write-once, once you write data to the tape it can't be changed or erased.