Probably because trains are limited in both weight and volume compared to ships and also less efficient. If you have this short route and know it'll need this amount of cargo shipped it likely makes sense.

This single ship can carry more containers than any train could be expected to pull, likely by at least one order of magnitude.

All in all I'd guess the advantages are roughly:

• Reduced staff
• reduced energy use (land based shipping is less efficient almost by default)
• no need for infrastructure except ports (if you assume there is no train line or this shipping would move existing lines over capacity building this ship is likely cheaper or at least in line with 300km of rail)
• simpler logistics (loading / unloading)

Disadvantages:

• Speed (a train would likely move at 3-5x the speed)

I would also not expect the risk for catastrophic fires to be all that high. This ship has the batteries be containers. So once you've designed a container that is a large battery, you've already spent so much that a proper BMS including proper battery fire suppression as well as proper breakers/contractors are things you've built into it without even thinking about cost. The separation provided by building containers as the battery is the next line of defence if one container fails spectacularly, it also allows the batteries to be maintained on land, much cheaper than if they were part of the ship.