Re: "Maintaining offline backups is expensive and a lot of boring, repetitive work."
Not forced - it was the logical choice at the time in terms of cost, capacity, and performance.
From memory back then, DLT was horrendously expensive for both drives and media, SLR (correction to above, it was SLR we used for our weeklies, not DLT) was similarly expensive. Smaller tape formats (the mini version of the SLR) didn't have the capacity or performance. DAT apparently met our mix of constraints - for a while at least - and had a conveniently small media size which helped with storage.