I think probably because "uh-huh" can have many different meanings based on inflection! As a "non-word", it relies heavily on how it is conveyed. Imagine some

I think probably because "uh-huh" can have many different meanings based on inflection! As a "non-word", it relies heavily on how it is conveyed. Imagine someone asks you a question, I bet you can answer using just the word "uh-huh" but conveying these different emotions: rude, perky, bored, upset, annoyed, dubious, excited and probably a dozen more. Even using the "perky" or "happy" one in a situation where it isn't warranted might sound rude or unthoughtful!