That's the thing. They did it for DAI (different people I know, but still) so why not ME? People wanted to know what happened to Solas, especially if we romanced him and then we are told the quest was cut, but a DLC was made to give us a true and final ending. It was the Citadel DLC of DA and that goodbye was a happy one with your Inquisitor and their lover together with all their companions, despite the fear of the Inquisitor's death hinted at throughout the whole story.
Maybe they didn't want an ME3 repeat? I heard it was terrible. I myself only got into ME last year so I'm not as affected as others, but I can understand how they must have felt.
Well, I think it is a realistic thought that the Trespasser-DLC and content is partly a result of the ME3-backlash. No proof on my side, but I think it makes sense...
However, in ME3 they were already done and done with their DLC-plans and all that, they never could have done a true ending-DLC (with gameplay and all) just to salvage their misjudged ending-experiment and please the rabid fans like me ^^
Tresspasser: I haven't played it yet but it seems it already proofs one thing: Devs CAN learn and listen from fans, at least for future projects ? Too bad Shepard had to pay the price by ending her adventure with a friggin gameshow and a ghostly boyscout as moderator, instead of a firework