Earlier quoted context omitted.
I somewhat doubt that a hollow plastic tank is significantly more expensive than a concrete block. Factor in the added cost of shipping the much heavier concrete block, and it seems like the manufacturer would also save money in the process.
Is it that much more expensive to ship something in bulk with additional weight? I honestly don't know how distance shipping by the container works, but I always thought it was just a case of being billed based on volume.
Note: there are a few different container sizes, but 40' is by far the most common. Weight limits also very by location.