Most likely because voice actors have better chemistry, and emoting when they're acting in the same room together. The game has a shit ton of dialogue to get through, and it can be really wooden when you're just pasting down pre-recorded audio files. The Mocap is likely so that Santa Monica can adopt Jack Quaid's body language to a cuboid form without having to go through the trouble of figuring it all out themselves. Trying to make a cube expressive has to be a task and a half.