Beware this is my personal view and no "factsheet".
Simplest reason of all:
The TP fee. Its not too high, its not too low, just because it is there.
Why, would you ask, you pay TP fee for CP and CND, no? Yes, I do.
But why would I pay those TP fees?
Because CP and CND have -special- things to offer for my gameplay. CP with fast hitting Aurlis kind of evade building and challenge my midlevel hunting "how do I do up there?". CND, pack up some amps for gamble mining and increased skill building. That is what I use those places for and why I pay the TP fee when I want to go there.
edit: An increased (well, admittably to a certain amount) or lowered fee would not really affect my travels there, because both places I consider "high cost" areas for me and therefore a visit there is some kind of special occasion for myself. So the reasons why or why not visit are due to other reasons.
At this moment, RockTropia has nothing to offer (for me) apart from other mob models. That is why the TP fee is not worth for my gameplay to pay for.
And it adds up that all Axes look the same for example.
Just been thinking this morning to take a trip to RockTropia, then thought of it why I haven't been there since my first visit when it started and then thought of reasons for it.