Why not multiple Bluefin-21's
There is only one Bluefin-21.
The other Bluefins 12S, 12D, 9M etc are mostly for shallow water work.
The Bluefin-21 is not the only commercial Autonomous Underwater Vehicle (AUV) equipped with side-scan sonar.
The US Navy has spent $20M on an order for eight Knifefish (a Bluefin-21 derivative) but these are not due to enter active service until 2017. They are for mine-detection and so (I surmise) may not be well suited to deep-ocean usage.
The Royal Navy has a number of REMUS 600 AUVs but these are not designed for use deeper than 600M
It may be that most existing AUVs equipped with side-scan-sonar are not capable of deep-ocean use.