Valve's Erik Wolpaw has said one of the reasons Chell doesn't speak in Portal 2 is because she is a silent protagonist which "could be anyone."
"[Chell] could be anyone," Wolpaw said during a NYU's Game Center talk. "It could be you. There's the silent protagonist, which I don't know is specifically a Valve thing, but it's a thing. It's probably a little bit tougher in Half-Life, because it's slightly more realistic and there's conversations that are happening and you kind of have to write around why Gordon isn't saying anything.
"I don't think people are super invested in the character of Chell...obviously because we haven't given her much character. But they're invested in the relationship they have as the player. We always assumed she could talk, she just chooses not to, what with robots all being dicks, why give them the satisfaction?"
Chell is also used as a bit comedy in the game, especially because she is more of a bystander in the games.
"There's this thing with comedy...there's sort of two different patterns," said Wolpaw. "One is, you're the straight man in a world gone mad, and the other one is you're a crazy person in a straight world. Portal is definitely the world gone mad straight man, and the straight man is you. Because you have to write in the margins in a game, time is at a premium, so the fact that there's already this established thing where you have a silent protagonist, that saves us a lot of time.
"You may want to know Chell's backstory, you may want to hear her say things, but I guarentee, if she had to say her straight man lines at the expense of half of the other dialog, it would suck."