It's mostly a critical mass problem, as far as I see it. If everyone had a different more positive tone, the one bitter one would stick out (and mostly people wouldn't even think about choosing that tone in the first place). But since that's what a lot of people do, that becomes the energy-minimizing approach to take. I suppose the best thing one can do is to have the courage to go against the stream ("vote with your feet") and find a way to make your presentation optimistic and happy (e.g. Julia Evans), despite the fact that you know there will be bitter people who judge you as cringy.