Because he's a protagonist with a player defined personality, and those characters are inherently boring. "Well what about X character" That character is the exception.
Let me explain my thoughts on it.
Basically, when a player has the ability to control a protagonist, the character will suffer for that accessibility. In that, the character needs to be somewhat unoffensive so that people don't hate him. The way bioware tends to go about this is to make the character boring. This is also common in other protagonists, and even some secondary or tertiary characters (Jacob).
Rarely do you see a character that has serious dialog options, that's actually good. The notable exceptions to me being Lee from TWD, and to a lesser extent Jensen from Deus Ex Human Revolution. So, I don't see it being an inherent issue with dialog options, I see it as being easier to just make them somewhat bland so most people will just be okay with the character. It's because of this that I have yet to seriously like a protagonist from a bioware game. You generally can't characterize them well.
The lack of being able to seriously characterize yourself is an issue. I've long thought that Bioware needs to do at least one conversation per game with your LI/Bestie that allows you to really fully establish your characters' background. Where instead of you asking your companions about their past, they ask you about yours. They did this somewhat with Josie in Haven, but it was a minor side conversation, and it wasn't quite as fleshed out as I would have liked. Certainly a step in the right direction.
They did however do a very good job at allowing your character to voice his or her beliefs on mages, templars, and religion. You can even potentially create a small story arc for yourself, though characters won't mention it. For example, starting out as a fervent believer, and then having a crisis after the events at Haven. Or starting out on the fence about the whole thing, and then being convinced by Blackwall to adopt the title of Herald not for yourself, but for the people so they have something to believe in.
Also, to seriously characterize the protagonist you'd need to basically write three characters, but have them fit together into one. Meaning, you'd need to write the most in depth character in the game, and have all the dialog from the different personalities be able to flow naturally through out a conversation. It would require good writing, acting, and arguably the most important aspect would be direction. To insure that the character would flow between one mood to the next, without it seeming jarring would require very good directing. Simply because you have a third party (the player) as a variable, who can change the tone of the main character at his or her whim.
The actor needs to be able to emote, but he also can't be too expressive because it might catch players off guard and result in something unexpected for them. For example, femshep at Tali's trial caught me off guard with the response "I'm not a lawyer" she screams it and gets very indignant. Whereas Maleshep seems to be more annoyed and a bit nervous. Which is more in line with what I was expecting. The tonal emoticons, or whatever you'd like to call them, are very important and should theoretically, allow for more emotion. Though, it would be nice if they had a quick tutorial at the beginning that would actually tell you what they meant. (A quick digression, I think Deus Ex HR handled the dialog wheel the best. An adjective to describe the tone, and if you hovered over the option it would tell you the dialog. Bioware should implement that.)
All and all, it's a balancing act.
I think the closest they got was Hawke, mainly because the "persistent personality" system really did make him feel different and unique depending on how you played. It's a shame that they dropped that feature for DAI, because it was one of the very few positive changes in DA2.
So, in short, it's hard yo.