Yeah, definitely.
Leaving aside the weird way in which some of the Bozja stuff seems to be actually implemented under the hood, there's the fact that Delubrum's easier to handle the queue for because you need only 24. (And Delubrum savage has 48 people but doesn't have a queue; you must pre-assemble your group and then go in, as there's no matching.)
So how do you handle Castrum queues? Do you wait for 48 people to be ready, thus a full run? But as it is, you often have runs with only 24 or 28 or whatever people, which are still perfectly viable; down the road when Bozja dies down a bit, this could be a problem, since if you wait for 48 people you could have 47 in queue for an hour waiting for that last person. So you probably end up needing to do something like pop an instance immediately if there are 48 players, but have alternative logic where if there've been 24+ people in queue for more than 10 minutes, you pop an instance with just whoever is in the queue rather than still waiting for 48 total. Or something like that.
Which is logic wildly unlike any other queue/matching and duty instanciation code in the game, so far as I know. On a wild guess, the headache entailed in figuring out how to make that part work is probably the sticking point.
What I honestly wish they'd do in the meantime is a) add a viewable timer on the Southern Front showing how long until Castrum is up (so you don't have an endless stream of people joining and asking "Anyone know how long until Castrum?" every 30 seconds), and b) every time you queue for Castrum and don't get in, you should get an item like 'Castrum siege plans' or something. And if you have 3 of the siege plans (I.e., have queued but not been picked three times), you can trade in those three siege plans for a Marching-Orders-type buff specific to Castrum that guarantees you entrance on the next one you queue for.
That way people can tell how long they'll need to wait for a Castrum instance to pop, and if they get bounced three times they can get a consolation prize of guaranteed entrance on the fourth time. It'd help smooth things over until carving out CLL and making it something you can queue for individually is feasible.
While the Arsenal is technically a step of the Eureka relics (i.e., the final step), it's also at the very end of a grind that most people weren't going to go through, so there's presumably far less demand for it to be able to be queued into separately.
The Castrum as it stands is is the equivalent of putting the Arsenal at the end of Anemos, and demanding you finish it before being able to move on to Pagos; while that might have spared some people the pain and suffering that was Pagos' map design, it would probably have produced far more outcry.