There doesn't have to be a lore explanation for everything...

... chances are it's just a result of currency inflation. 100 gil is the cheapest that can be charged for a teleport. Currency values inflated hugely over 2.x, while the porter prices are set to 2.0. So people have more money to spend, naturally they're going to inflate the prices on porters.

If you want a lore explanation, let's go with the "it's dangerous and uncharted territory" SE used to explain the difficulty jump.