Its because it cant be discussed sensibly any longer.
You either believe and agree the world is about to end and that life as we know it has to change immediately or your some sort of climate Nazi.
Id wager the vast majority are worried, do want some changes and already do contribute their bit ie recycling where possible, use of public transport, cycling, walking etc instead of using the car. Have cut down their meat intake etc etc but for a small very vocal few it isn't enough and rather than being encouraged they just get called silly names and like much of today's political debate, fucked off with it all.