because most of the world contains shallow narrow minded people who only give attention to what they can see. EG, I have explained many times to various people that just because you can see a puff of smoke from a diesel, does not mean they produce more emission's than a clean 'looking' petrol (which could use 2x the fuel!). In addition to this although its easy to die breathing petrol engine exhaust, you dont die from diesel exhaust, but you may feel sick. This is contrary to what many people believe based on what they see.

BTW if you dont want to gas yourself with harmful emissions take a cold shower.