It's Paramount trying to keep interest in the films going by keeping the original film aesthetic as intact as possible while also trying to please Hasbro who is going full G1 with everything. Technically they did with TLK, but it happens in the BB film probably to explain why he can't (or isn't supposed to because he does at one point) talk in the first film. If they decide to go full break-away reboot with BB, I bet his voice gets fixed much quicker since nothing would make the films seem different than a talking Bumblebee for a change. Spoiler: There's a reason for that Bumblebee loses his memories early in the film, after he gets his ass kicked by Blitzwing, before he meets Charlie, which is why he's acting like a scared puppy. He later gets his memories back before the final fight which allows him to get his weapons systems online again. His battle mask isn't exactly friendly looking, though, which is why his default is the more rounded, human like face. It makes it easier for him to emote with a humanoid facial structure which given he can't talk is a much more important facet of his animation. The film wants to be a more kid-friendly transformers action film aiming for the family demographic, instead of the young teen/adult market the last five have.