So there are basically two types of silent protagonists: the protagonist as you, and the protagonist as someone else. The protagonist as you is the Fallout model where they are basically you, you control them and they say what you want them to say. They are silent in the sense that they don't have a personality, they have you. You do the talking and the protagonist is just your vessel. The interesting one is the protagonist as someone else, because why don't they talk?
It is worth noting that I'm not talking about the strong silent type, who just doesn't like to talk. I'm talking about people who say literally nothing the whole game. Most confusingly, no one mentions the fact the main character is mute and so the end result is that is seems not so much like the main character doesn't talk but rather like you never get to hear what they are saying the whole time. It is a very strange video game thing that happens in a lot. The idea is that it allows the player to better project themselves into the game as the main character. The less they talk the less they says things that make you hate him and not want to be them. The problem is that games that do this tend to be RPGs with big stories, and you can't have a story without an arc for the main character. But how do you show that character arc when they don't speak? While it does have the benefit of making the character easier to project into the fact that it drains the focus of your story of personality is a huge handicap.
A game that does this really well and that I absolutely love is Chrono Trigger. Crono doesn't say word one in this game and that is never commented on, but it works. Why does it work? Because despite the fact you never hear him talk he is still full of personality, the game just doesn't use dialog to show that. It shows it in spades though through his actions, his relationships with other characters, even though the battle mechanics and the skills he learns. He still has an arc; he starts out not even able to get out of bed on his own but goes on to accept increasing amounts of responsibility until it becomes too much and he has to pay the ultimate price. Only to be saved by his friends and realizing he can depend on them and that he doesn't have to do everything himself as evidenced by the fact he is now not a required party member. That is an arc, that is character growth and he didn't say word one.
Does the fact that he never speak mean it is easier to project into him? I guess. But the important thing to note is that the removal of dialog was replaced by giving him personality in other ways. Removal of dialog as a technique shouldn't mean the removal of personality (as it is often treated as), in fact you have to amplify and reinforce that personality in other ways to offset the fact you never hear them speak.
Fallout games get away with not giving the characters personality because the player gives it personality, they are the ones making the decisions and whose actions are speaking. But in games where you don't give the player the ability to set actions, where the story is on rails, you have to give the main character personality or else while you are giving the player a point of entry into this world but you are giving them no reasons to care about it. Crono cares, and as he is our point of entry so do we.