With the US blockade on Iranian shipping and ports in force along the whole of the southern Iranian coastline, and the railway link to China through the border crossing point with Turkmenistan at Inche-Buren damaged, the route across the Caspian has become the only cargo route for heavy traffic which remains under Iranian sovereign control.
Iran has been supplying Shahed drones to the Russian military since the start of the conflict. Ukraine told them to stop supplying Russia with the very weapons that enable Russia to strike their cities.
Ukraine didn’t go out of their way to start attacking random ships or murder Iranian citizens, or even attack their infrastructure. They sank a ship carrying weapons that Russia was intended to use to hurt them. That’s pretty justifiable.