Because I have actually read the text and comprehend it.
You know how some people have to read along with the audio, like they'll get an audiobook and the actual book and read it that way? Well I can do that of course but I have found that it's better if I just pay attention to either 1 or the other, not both. And if the quest dialogue is up on the screen while they speak I will often read it anyway and then the audio trips me up and I end up not understanding any of it lol.
Now with this change to turn off the quest audio it is more like how I grew up with gaming. Ironic, because ESO's voiced dialogue is one of its absolute best features, I whole heartedly recognize that and it is awesome. But this is like one small secret change to make it more tailored to how I personally actually like it, instead of going with how I think it is supposed to be experienced. It's just ironic because it is a reversion, or stepping back in time almost in terms of gaming "quality" and I like it more that way.
Anyway, gonna continue with it like this for the weekend and see if it's a change that will stick. It would be cool to have the option on the quest dialogue screen itself, to play the audio, even though my dialogue audio is turned off, if I wanted to. Instead of going in and out of settings for that.