Well the reason why that is, is because Javik. Javik feels like cut content because he is cut from the original story. His character was originally what would've gave the Cruicible the big red button that would destroy the Reapers. Javik is the original Cataylst and that is why he feels like cut content and feels so essential to a lot of backstory. He gives tons of backstory that people who didn't do the preorder on ME3 or buy his dlc seperate will never experience. Javik is probably why the ME3 endings feel so messed up to some people like IT believers. I believe in IT but I probably ain't a true IT person since I open to other interpretions and thats because endings are so controversal. But back to Javik, I bring Javik up because a group of friends and I were having a conversation on Javik and those points were brought up. But then I think upon the original script leak of ME3 that we saw over the summer before ME3 released and to be honest it all makes sense if you think about. Javik and him becoming DLC is what messed up the original intent of ME3, Javik is loaded with essential background and you learn alot from him.
The problem which occurred with ME3 is that you just can´t release more story driven DLCs to fill blanks, and if you do then you can´t be surprised if players wants to crucify your ass. Javik´s DLC didn´t messed up a narrative, because story has been changed from the Dark Energy arc into the Starchild concept. However it messed up only the model just by simple fact of being behind the additional paywall. You just can´t cut content which is connected to core of the story, while Javik had not much of addition in meaning of core narrative of ME3, he was still part of last two previous games which were focused around the Reapers and Protheans, as a central plot flags of the story. If the literal version is true, then on a scale from 1-10 they much more messed up narrative by Leviathan as much as again attacked the integrity of own product. If we had Leviathan as normal part of game then none would probably even question the outcome, even if players would be unhappy or angry about different things, they would get at least narrative explanation. The thing about DLC is that I don´t mind eploring the world within the universe in which is set the story, but I have a problem with exploiting of central themes thru DLC. The key problem about Leviathan is as I mentioned is that it does not explain Crucible, despite the fact that game try so hard to explain it and ultimately failing in it.
The first is the archive on Mars that gives you nothing more then it´s supposed purpose and design, second one is Vendetta who has informations at some level, but it´s questionable if the VI knew how it does work or how it can be operated. If it´s true that Crucible was created by some random specie, then it makes sense that only Vendetta can and know how to use Crucible, because the information about it came from previous cycles and thus someone at some point realy found a way how to destroy Reapers. Story explicitly even says that none ever made it to Catalyst, none ever managed to use or finish the Crucible and all of previous cycles were destroyed right from start with invasion when Reapers shut down the relay network. The last one begs a question of how is even possible that someone could even create Crucible which is supposed to hack entity that none ever knew about, well unless it was planted by Leviathans or Catalyst themselves.
The third explanation is said by Leviathan, which is probably lying about the Crucible´s origin, because there is this strange moment in which camera view focuses on Leviathan´s pawn eyes within the dream sequence, that implies either way doubt about it´s purpose or simple fact that Leviathan is trying to hide the true purpose and origin of Crucible. What´s quite interesting about this scene is that what Legion has mentioned within the geth consensus, when he tried to portray things to you with the reality that you are familiar with. If this is true, that everytime within the world of ME, that someone is trying to create an illusion of being in some sort of dream or virtual
reality thus some kind of communication channel, that this reality itself may shape things but can´t hide lies within, only to paint them with something that we are already familiar with.
Last attempt to answer is made by Catalyst or the entity that claims to be Catalyst. The thing with Catalyst is, that while Vendetta claims that Citadel is Catalyst, the entity claims to be the Catalyst. While the entity claims to be a Catalyst thus Citadel, Leviathnas claims that entity - an AI created Reapers and thru them Citadel and Mass Realys, which implies that AI at some point had to be based somehwere else. But if existed at Citadel whole time, what was point of having ME1 or what´s the point of Prothean´s sabotaging the keepers if the whole time was their true master on the Citadel or what´s the point of having the keepers in order to activate the relay from dark space. Well, not unless the Catalyst is being red herring and the true identity of an AI is Harbinger or something else entirely.
The problem of those two claims is who managed to contradicts itself and lore more, and the entity is simply winner. While Vendetta has been only reproducing only what was stored in its memories, Catalyst or the entity contradicts the story and even itself by answering the question about Crucible. It´s even said that Crucible is supposed to be nothing more than battery, if it´s true then choices are brought up to the table by Catalyst, they are even based on the Citadel and that´s one more problem of whole claim that Shepard somehow managed to alter the Catalyst. Catalyst had no obligation to help Shepard, to uplift Shep with that platform, or to even talk with him and he even demostrate it with simple fact of letting you to refuse the choices given by him.