I have always wondered: why do adults in Peanuts not really talk?
Why do they only make that wah wah wah noise? (By the way, what was it? That sounds like a trombone or a trumpet with a mute.)
Television producer Lee Mendelson was already a few years into his collaboration with Peanuts creator Charles Schulz when he was faced with an unusual problem: How do you depict adults in an animated special for a comic strip that never features adults?
Mendelson told Mashable, “We chose not to show the adult. So I asked our music director, Vince Guaraldi, ‘Would there be some instrument we could use as a sound to emulate what an adult might sound like to a kid?’”
Guaraldi, who first collaborated with Schulz and Mendelson on a 1963 documentary about the comic strip legend, didn’t hesitate. He brought in a trombone player. (mashable)
I know that this is a question posted regarding the TV Show and, potentially, the recent movie but in regards to adults in Schulz work, the Peanuts Wiki site states:
Schulz said that grown-ups just did not interest him. The absence of adults also gives Peanuts a unique point of view, bringing the comic down to the level of children, and not children from an adult's viewpoint.
A few years or so ago I wondered this same question. I can't recall where but I do seem to recall that I managed to find a reasonably reliable or "official" source expressing these thoughts nearly verbatim. Likewise, you can find similar mentioning's of Schulz perspective on adults in his strips throughout the web.
The bottomline from what I've gathered in my research is that having adults express themselves within his comics would have provided a form of "noise" that would have detracted from the many adult moments that were being communicated through the experiences of the children within the strip. Peanuts resinates with so many people because through the simplicity of children, we often experience very deep and emotional observations of life, ourselves and others.
If this was legitimately Schulz intent then having adult dialog would have degraded the essence of the messages he was trying to express.