I tend to implicitly associate unicycles with performative comedy. Probably due to a lot of circus and talent-show imagery. If I only had a split second to react to a unicyclist passing by, I might automatically assume that the rider is trying to be funny on purpose. And therefore (my subconscious leaps ahead) that the appropriate social reaction is to respond in kind with whatever humour I can come up with in that split-second, which will probably be an unoriginal and poorly-landing joke.

Ten seconds after saying it, my more considerate thought process will probably realize that the joke wasn't very funny, that the rider probably hears the same "joke" a hundred times per day, and that they might have just been enjoying their solitary unicycling hobby, rather than deliberately putting on a show to amuse the neighbourhood. But by that time they'll have long since passed by, and it would be too awkward to catch up and apologize.

Honest attempt at humor perhaps. Recognition of the absurdity of the act. Camaraderie done poorly.