After spending a day with an older man who hates everything outside of his comfort zone, I can say that many people probably just feel uncomfortable thinking about earth's problems and the people who are trying to do something about it.
It is much more comfortable to sit blindly in life and be mad at people who think outside of the box. Or, put them down and make fun of them.
Moreover, a lot of people choose to believe stories that they hear from the news or powerful companies, and get mad when someone (like a person who is trying to keep the Earth healthy) contradicts their beliefs.
Lastly, a lot people think hippies are stupid and clueless, and because one person calls them dumb, other brainless sheep have to call them dumb as well, or they risk stepping outside of general opinion.
In short, because of ignorance and stupidity.