The big question is scale: tape robot systems are expensive to purchase and somewhat less reliable compared to an HDD so you have significant infrastructure costs bringing up the storage system, establishing a system of off-site rotation and redundancy, testing restores on separate drives (one common failure mode is that a tape drive slowly goes out of alignment, writing data which cannot be read by a calibrated drive), etc.

These are great examples of something which could be commodified so a provider could amortize them across many customers but the margins are continually tightening.