Nunca é explicado e até mesmo os criadores da série parecem ser
We inherited Bumblebee’s speech handicap from the Transformers films: that’s the Bumblebee that the new generation of fans knows. That said, our backstory for how he lost his voice stems from ‘Exodus’ continuity, though it will be a while before we see that in the series.....But, with Raf and Bumblebee, there’s an instant mutual rapport.
De acordo com algumas anotações em seu diário ("não é um diário!") parece que nem o próprio Raf sabe o porquê;
"Bumblebee's my buddy. Mostly cuz I'm the only kid who understands him. Not sure why ... he makes perfect sense. He just doesn’t use words. Might be the same reason I could understand digital code, even when I was three years old. Anyway, most people talk too much. Best thing? Bee's an awesome muscle car, and he’s my friend."