Well, a possibility for boat > airship could be that all Airships are Ceruleum powered. Now if you take the entire mass of the Garlean Empire, you can imagine just how much cargo/supplies/etc would need to be shipped around. Considering the world's largest deposit of Ceruleum is in Northern Thanalan (a region the Eorzean Alliance has now taken back control over), unnecessarily wasting limited Ceruleum resources on the transport of goods may not be practical when Ceruleum also powers nearly their entire military might (Magitek).
So weigh the options: faster and safer delivery of goods, or not have enough fuel for your war machines?
Considering Garlemald also has an impressive navy, it makes more sense to send goods by land and sea instead of air, even though that puts their goods at slightly greater risk. Take into account, the Rogue storyline doesn't mention just how many Privateers may fall attempting to knock over an Imperial vessel.
So weigh the options: faster and safer delivery of goods, or not have enough fuel for your war machines?
Considering Garlemald also has an impressive navy, it makes more sense to send goods by land and sea instead of air, even though that puts their goods at slightly greater risk. Take into account, the Rogue storyline doesn't mention just how many Privateers may fall attempting to knock over an Imperial vessel.
