I'm not sure if they've ever stated their reasoning, but I'd imagine it was to illustrate that the opening scene was set in the distant past on Earth, and having a more stylized look for the future just works better aesthetically.
"The algorithms I have been programmed with do not support the comprehension of illogical human thought." ~ KOS-MOS
GS (Gamespot): In the first portion of the trailer, the character modeling looked more realistic compared to the anime-style modeling in the remainder portion. Why is there a difference between the two?
TT (Tetsyua Takahashi): One of the reasons is that the two different scenes take place in different time eras a few thousand years apart.
GS: Will the game shift between these two time eras?
TT: Basically, the later portion of the trailer is the main story. The first portion will only be used once as an introduction. But if we do plan to return to that time era, we will try to use the same models as much as possible and make adjustments upon making new characters.