In Pokémon: Detective Pikachu (2019), Pikachu got merged with Harry, enabling him to talk. When Mewtwo merged Psyduck with Lucy, Psyduck still couldn't talk. Why can't Psyduck talk?
It's not so much that Psyduck can't talk, it's that no character has enough of a bond with Lucy to hear such a voice. At least this is what this interview of direct Rob Letterman hints at:
ET: What sort of conversations did you have in terms of determining the rules of that? Insomuch as Tim is the only one who can hear his father within Pikachu and when Lucy (Kathryn Newton) is put inside her Psyduck, she still talks as Psyduck.
Rob Letterman: I mean, the father and son thing is the whole movie. That's the heart of the whole design of the story, it's all curated to be towards that ending. And the conversations about that, you know, the real reason Tim can hear Pikachu is not-- There's no science. It's not the gas. It's not Mewtwo's superpowers. It's purely the connection between father and son. It's the bond. It's the emotional, spiritual thing. That was one of the first conversations we had with The Pokémon Company is that that father-and-son bond goes beyond anything and that's why he's hearing it. But we had to hide it in all these other little misdirects, so that we didn't give away the ending.
Tim can hear his father's voice because of the bond they have - they have grown estranged but the whole theme of the movie is that bond actually being very strong. Tim likes Lucy a lot, but does not (yet) have a comparable bond with Lucy; hence why all he (and we) hears is regular Psyduck talk.