[2bitencryption]

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!