Because it's just so fucking depressing, and it's one of the first impressions the game makes once it's really begun. While you're playing the game, shooting nazis in mech suits with over the top weaponry, mixed between weird comical shit going on at your base, BJ's narration is at odds with the rest of that because he basically sounds as if he has a gun to his head, writing his suicide note. Don't get me wrong, the writing in this game is great, but BJ is nothing but a wet blanket for the first half.
This is all intentional though. BJ is carrying the baggage of the last game into this one, into a stalled American revolution where the few resistance fighters left have a completely different vibe than he was used to in Europe, where most of the country has been firebombed and nuked to hell, and where the events of the opening have taken place. He's a broken guy in the middle of a a whole bunch of people operating on different frequencies and feels like he can no longer keep up. He also GOT KIDS ON THE WAY that he knows he won't live to see. This is all in support of building to the mid-point of the game.