Not an expert, but I suspect that getting them from the factory to the dock (and finding a dock that would not be a nightmare to maneuver through) is a pretty significant part of the challenge
Loading them on boats is expensive because they are non-standard size, and large and flexible enough cranes aren’t available everywhere.
Finally, there aren’t boats of the right size, and those that are increasingly expensive. Airships are also faster (not by much, admittedly) and less sensitive to bad weather (not by much either) in particular waves.