Because that was the day the dark nature of the universe was completely rendered null and void. What made Origins great was that no matter what you did, in the end, someone had to die. The only choice to invert this was letting Morrigan (the morally questionable party member) run off with a demon god baby. No matter your choice, there was a consequence, and it had actual weight to it. The only way you could cheat death was to allow Morrigan and her already proven to be extremely questionable decision making skills try to raise a god baby by taking a corrupted soul out of an Archdemon.
Pretty deep shit. All of your choices had consequences. The only way to save yourself or Alistair (or Loghain if you are running a
complete monster run) was to allow Morrigan to potentially recreate the same horrifying problem some years later down the line. Because remember, even if Morrigan was trustworthy (which she demonstrated several times over she wasn't even when she liked you), she demonstrated a callous disregard for human lives in multiple instances, and might have been totally overwhelmed by the power she was trying to raise and/or control. In the end, the game states (if you choose Morrigan baby ending) that she runs off and abandons everyone she knew in the party
no matter how much she might like or dislike them, telling everyone not to follower her.
This leaves uncertainty in the only ending that doesn't require the death of a major protagonist. This is how dark fantasy universes function at their core: All choices having consequences, whether known or unknown, that never allow a perfect world. You can have lighter or darker shades of grey, but it's always grey.
Morrigan's DLC changes that by allowing you to meet her and allowing you (the protagonist) to subvert that ending. Don't like it? Just murder her now. Think she can do it because the plot makes no indication that she can't? Go jump into the portal with her and be a baby daddy for the god baby. A baby daddy that, for some reason, Morrigan now suddenly wants to have along when she originally ditched them in the first place.
#1: It destroys Morrigan's character, in that she went out of her way to get as far away from everyone as possible to raise her God baby, and then suddenly changes her mind at the end and offers to take you along.
#2: It makes no sense from the perspective of the original ending. It was suggested that Morrigan had already long escaped before you could even start looking for her, but somehow, you find her anyway.
#3: It allows you to erase one of the most major consequences of the previous story. Which might not be so bad, if there was at least a negative consequence for it. Aside from guilt for murdering Morrigan, there... Really isn't any. If you're even slightly sane, you should choose to end her life and not risk the entire world on what is by every definition a mad gamble on her part.
#4: How do you even try to write for this in the future? The answer is you really can't, you've passed beyond the human power threshold. You can only retcon from here on out.
And then Dragon Age 2 appears, and repeatedly retcons things you did in the original game. The ending is also identical no matter the choices you make, rendering the consequences of your choices null and void. (Whether you side mage or templar, the mages always lose, and the templar commander turns into an evil power ranger.)
And then Dragon Age Inquisition appears, and has Leliana in it even if you directly murder her in Origins. Reasoning? "I got better." From death?! Are you kidding me?
The reason why the sequels keep coming across like shittily written fan fiction in terms of the plot (even if the characters are kind of interesting in their own right), is because they're all trying to follow in the footsteps of Origins. Which is a problem when Origin has so many decisions which do actually impact the world at large, that trying to account for all of those decisions is extremely hard. From the PoV of a writer, I can very safely say that I'm not proposing I'd do a better job. Au contraire: Nobody could fucking do this within the confines of a video game. I could only imagine doing something like this, ironically, with the liberties that I get from role playing, since nobody has to program my writing into the game, and I only ever have to follow one particular path the players choose and can disregard all the others.
Morrigan's DLC, however, is where the beginning of this problem appears. The writers attempt to give you a way to retcon the only consequence of the Morrigan baby ending, thus completely rendering null and void the dark nature of the world. Once that's gone, there is no more serious consequence or friction that can appear. In DA 2, mages and templar just start becoming more tense and angry at each other... Because they do. There's really no modus operandi for why the templar suddenly turn into major dickholes, they just do. There's heavy handed references to the Nazis (ex: "a final plan for the mages." Just replace "mages" with "jews" and there you go, have fun.) but there's no sense as to why the templar are nazis: Why are they doing this? Because they suddenly are paranoid of the mages they've essentially grown up with? For centuries? Why? The plot is incoherently written over an entire decade of history. In that decade you also have the Qunari, who... For some reason... Wait, like, what was? 6 or 7 years before they decide to enact their Qun-based city murdering blood bath? Any reason why it had to take that long? No? Cool. Red Lyrium which was once depicted as basically an instant death sentence for whoever touched it, can now suddenly be forged into a sword that power ranger lady can carry around for
years before finally going mad? And... None of her subordinates questioned the whole "red lyrium sword" thing? The mages want freedom now? Why? All the mages in Origins (aside from the runaways) seemed perfectly fine with the way they lived and understood why: Because they could become demons at any time. But now suddenly, not only do most of the mages want freedom, but they're willing to go to the extreme of invoking demon blood magic and turning into this world's version of the IRA. (Fuck you, Anders.)
The reason why the fighting in DA 2 seems so ridiculous and stupid is because they have no reason to be fighting at all after the ending of Origins. Morrigan's DLC only compounds this problem because the one thing that could have caused an issue after between mages and templar (Morrigan) is now gone. Any potential conflict that could have arisen as a result of Morrigan's god baby, is also gone.
DAI also has this issue compounded because even with an open green portal invasion by horrifying monsters that are indiscriminately murdering everyone, mages and templar still feel the need to fight each other instead of...
That. It totally wrecks the Grey Wardens mythos for no reason as well.
When your choices no longer have real lasting consequences and get rendered null by the plot, that's when the plot stops mattering. In a choice-based narrative, that's when the narrative stops working. That's why you might hear people talk about how much they liked certain characters in sequels after Origins, but pretty much none of them will really even remember the stupid plot.
