Wow I certainly didn't expect to see a topic like this so long after the game's release.
At any rate, Morrigan plays only a minor role in the game and comes in rather late. Also, her introductory mission is non-linear, meaning that the player can choose to play it sooner or later in relation to another mission. Even if they considered making her a romance option, there just isn't the time to develop any sort of relationship. And too, the devs don't just go about writing these romances lightly: they consider whether it would be appropriate for the character and the story as a whole; they just don't throw it in there willy-nilly.
And if DA2 was anything to go by, they typically did not have characters who were previous romance options as new romance options in the following game. It seems to me that they would rather just avoid the headache altogether. So, I also don't imagine that if we go to Tevinter in DA4 and a non-romanced Dorian is there that we will have the opportunity to romance him, particularly given his romance content as presented in DAI.
Morrigan doesn't seem to like the Inquisitor all that much tbh. But maybe that's just cuz I kept choosing the options that made it sound like I knew her.
She sounded like she respected me, so... I dunno. Then again, I tend to not choose inappropriate dialogue responses in general.