> tldr: it was a very radical idea and he was a difficult, unliked man. I read that as "kind of a jerk", but that's my interpretation. We might react the same way if the person you dislike for being a smartass comes with an idea that there are invisible tiny things killing people.

Semmelweiss didn't know what it was that killed people. He had some results that suggested a correlation between hand washing and mothers surviving. But he couldn't explain what the mechanism was. It was quite easy for the scientific establishment to dismiss his results as a peculiarity of the ward he was working on.