The developers at Platinum Games, who were huge Transformers fans since childhood, decided on the Generation 1 aesthetic of this game. In order to modernize it, in addition to the 80s G1 cartoon, they also took inspiration from the G1-themed Transformers comics by IDW as well as the Transformer: Generations toyline, which is also based on the original 80s designs.
Contributed by PirateGoofy