Originally there was going to be a third main character that would develop as the game was played in a Tamagotchi style. Instead Naughty Dog concentrated their efforts on two main characters in order to create the exact characters they wanted.
The reason why Jak doesn't talk in this game is because Naughty Dog wanted to enhance the experience and let the player truly feel like they were Jak.
This game pays tribute to the 90's games for example Super Mario 64 game and Spyro Trilogy
In Ratchet and Clank 2: Going Commando, Ratchet can be see playing Jak and Daxter: The Precursor Legacy in the title screen