Pondsmith stated in an interview about the game that he and the devs decided upon it also due to the fact that 3rd person would be metagamey if you can play with the camera angles and see more than just what's in front of you.Didn't they say they were gonna have some sort of "photo mode" to pop into 3rd person for screenshots or whatnot? Anyway, not surprised but still disappointed at these questions, no one asked if we can wear baggy coats that make us look bigger than we really are.
Why this lack of 3rd person anyway, "immersion"? It's a waste of customization options if you have to toggle some sort of freeze frame shit on just to see yourself.
I must say that's a pretty lame excuse, the real reason must've been some technical limitation. Hard to believe, but it must be. 3rd person better be moddable.