They gave Gears a toy-accurate head, but couldn't be bothered to do the same for Bumblebee?
The general guess is that Gears had that head pre-planed as an existing part of the mold so it was free to be used, but the BB mold didn't have a counterpart.
-ZacWilliam, I mean the Goldbug head is closer but also not exactly the G1 head, so shrug..