I don't know actually. I always thought that there wasn't enough storage space for full voice lines or they couldn't use any of the lines from the movie, but it might've been a design choice. If it was, it was a weird (yet oddly fitting) design choice.
Yeah it probably was a design choice, cause iirc the sounds the characters made were mostly unique for all the scenes, and they just make sounds similar to how they would be heard if they were speaking English, but with just the emotion. So more or less the same size.
It makes sense if you think about it, with the way the LEGO figures look, the solution that most would come up with is for them to speak non verbally, along with cartoon humour. It's more weird that they now speak normally, though I prefer that more sometimes like in the upcoming Skywalker Saga.