The main difference between ALL of the DA games and a game like Diablo is that the DA games are heavily cinematic and have a ton of dialogue. Both DA2 and DAI even more so since the protagonist is fully voiced. Considering the large amount of codex entries in Dragon Age games, it would be prohibitively expensive to have the codex entries be voiced.
Also, there is the issue of who would read which entry. Diablo III has journal entries that are read in each of those character's voices. There are lore entries read by Deckard Cain, and a bestiary read by that other guy. It's relatively straight forward with few reasons to have a large variety of people.
But again Dragon Age is different. Each codex entry is written by a different person. Or sometimes it might be something that is unattributed, or anonymous, or simply a text with no specific author. Who would narrate those? Perhaps the most commonly found person would be Brother Genitivi, but there are still the remaining numerous entries by a random person.
It seems at the moment that they only take into consideration the opinion of the people who play the game on Nightmare.
I have no idea what you even mean by this. The various difficulty modes only have to do with combat, which is only a portion of the gameplay in any Dragon Age game.
Also, there are different reasons for whether to take certain player feedback into consideration. The writers are going to look at feedback on the story and characters in a different way than the mechanics designers will look on feedback for combat systems. For one thing, they can do all the testing they like prior to release and/or patching, but there is no substitute to potentially millions of players actually using those skills in ways and combinations that they never thought of, including exploits and abuse.
In general, I think it is more important to look at feedback, and actual skill use from telemetry when working out patches and updating combat, and I think those designers should take that into consideration. But I do NOT think that the same applies to the writers. Story is different from gameplay. I also think it's an unfair statement as most of the non-combat feedback can't really be taken into consideration until the next game is in development.
Some of the remarks revolving around the player's exposure to Corypheus thought DAI are one example of this. I believe Mark Darrah (or Mike Laidlaw) did say at one point that they realize that the story could have benefited from Corypheus being a stronger presence. But there isn't anything they can do about it post-release. They can only incorporate that into whatever future game they work on.