I've long suspected that this is a case of scenario supply and demand.
Dalamud dropped quite a few internment hulks on Eorzea (especially if you try to count all of the ones you can see "in the distance" added as horizon flavor to other zones). By making sure that all of the main ones had obvious locations and ways you might get to them, they could ensure options if the need arose (kind of like how I suspect that they haven't released the world map even though they have a general idea of it just in case they need to tweak it to work with future ideas). Should this have been the case, they'd have ended up with more options than coils.
I was pretty surprised when Boulder Downs turned out to be nothing, but I was downright shocked that we didn't do more with the Mor Dhona hulk. Look at that thing. Is has its own thunderdome! And we skipped it!
Now, part of the answer is obvious: only four hulks survived the collapse of Dalamud to the degree that they could functionally burrow and re-align to find Bahamut's heart. The others are varying degrees of broken, and we can only speculate as to what (if anything) within remains functional after five-point-some-odd years without the systems that failed.
On the one hand, we can quote Koji Fox:
On the other, we have to consider Xelphatol Protocol: going back and opening up the other hulks is possible, but are we sure that it's more interesting than other options? Similar to how Xelphatol is mostly a bleak, windswept region with barely anything to see, its sacrifice might pave the way to something more novel or enjoyable. (Open-world Promyvion-like zone, please. Purple glows.) If Earth were an MMO, and we wanted to add, say, Spain, does that inescapably bind us to have France, Germany, England, etc. at the cost of India or Russia or Peru? We could see a little of so much, instead of so much of a little. Those are scales that must be balanced. (With open-world, purple Promyvion, please.)Originally Posted by Gamer Escape Interveiw with Koji Fox (E3 2016)