Because when you use adverbs as dialogue tags, you're telling not showing. You're
telling the reader how the character feels instead of letting them experience the emotion and figure that out for themselves while they read. You distance them from the character in that way as well.
Writing a novel isn't an exercise in creative writing. You need to put your extensive vocabulary aside and simply write the story. You want your readers to get into the head of your POV characters and live there until you change POV in the story. You want them to see what that character sees, hear what that character hears, touch what that character touches, taste what that character tastes, smell what that character smells, and feel what that character feels.
The point is not to tell them what they're supposed to be seeing, touching, tasting, hearing smelling, and feeling, or to impress them with how many ways you can use an adverb to describe speech sounds. It's to write in a way that allows them to experience emotions and sensations along with the character. You do that, and you have a reader who will not put down that book until they finish, and will be sad to leave the world you've created. They will buy everything you publish.
You give the reader clues about the mood of the speaker through dialogue and the actions surrounding it. Using adverbs as dialogue tags is lazy writing, and it makes for awkward sentences. Adverbs that describe facial expressions but are used in dialogue tags makes them read as though they are describing the sounds, not the expressions.
EXAMPLE:
Carol could feel her blood pressure rising as she listened to his words. Damn infuriating man. Why did she even bother? "So I suppose
now you'll tell me I'm not here so you can blackmail me. There's another reason."
His sneer made her want to slap the expression off his face. "No, I still intend to blackmail you. But we'll have some fun first."
AS OPPOSED TO:
"So I suppose
now you'll tell me I'm not here so you can blackmail me," Carol snapped angrily. "There's another reason."
"No, I still intend to blackmail you. But we'll have some fun first," he said sneeringly.
Which interaction gives you more of a visceral reaction?
In the second, all you get is confirmation that Carol is angry, which we already got from her words. It gives you nothing about her internal monologue or the conflict she's feeling even as she says the words. All you have is "snapped" and "angrily" to go on. It's flat and boring.
As for the man, again you get nothing except that he's sneering, which you have to work out from the adverb. It's an awkward sentence because people can't really say anything sneeringly. You can sneer
while you say words, but that adverb does not describe speech sounds- it describes facial expressions. Yet I see this type of thing all the time and it pulls me right out of the story as I try to work out what the author meant. When I see this, I have to wonder if the author actually understands the meaning of the words they're using in the dialogue tags.
Now let's examine the first interaction. We know exactly how angry Carol is but we know more than that. This man infuriates her. He makes her blood boil. And the addition of "Why did she even bother?" gives us something to wonder about. Who is this guy? Why is she conflicted about him?
We know the man is sneering but we get even more than that. She wants to slap him. Why? What's going to come next? How will she react to what he said?
Not a dialogue tag in sight in the first example, yet you know exactly who is speaking each time because of the internal monologue and action around the speech, and you know what the POV character - Carol - is thinking. You're feeling her anger and conflict right along with her, instead of having the author simply
tell you she's angry.
I agree that it's good to toss in a tag or some action or something if you have a lot of dialogue back and forth, even with two characters. I try not to do more than three lines each back and forth before I put in some action or internal thought, not so much to remind my readers who is speaking, but instead to let them feel what's going on as the characters are talking.
IMHO, if you've done a good enough job with the characters' distinct personalities, you don't need to give readers clues who is speaking. They will know because of the words and the different speech patterns. None of us speak the exact same way. We each have different pet phrases, unique ways we string words together, etc. Give your characters those same qualities and it will be obvious to your readers who is speaking.
Hope this all helps.