It might be due to the fact that it is about Wikipedia.
Most people have strong feelings about Wikipedia because it has become a big part of many people's internet experience. And people usually don't like change and can become offended when people propose to do so.
You are onto something. I wasn't a huge fan of the redesign but I couldn't understand the high emotional tone. A lot of the feedback runs along the lines of 'how dare you change Wikipedia without going through the Wikipedia process', etc.
Most people have strong feelings about Wikipedia because it has become a big part of many people's internet experience. And people usually don't like change and can become offended when people propose to do so.