Because a large number of people on boards like Reddit or Twitter who call themselves centrists use “I’m not a republican” as some way to excuse bigoted views on race, LGBT issues, etc. I suppose the opposite may be true if you’re in a right-leaning place and have left-leaning views, although I wouldn’t know.

While that might not be the view of centrists in general, that’s the perception of the “centrist” on the internet, and why people generally don’t like them.