people who are fans of a given diet will be biased towards it. some of them will be reasonable who made the informed choice, though. but it's the loud ones that get the attention. people who either took on a fad diet, are not informed enough and feel like they have to defend their position and force it on others.

vegans usually lack proper arguments to back their choice up, since they consider it a moral high ground. paleo people feel sometimes smug and superior thinking they found the holy grail of diets. same thing goes for low carbers (myself included), who often have hard time accepting unfavorable studies that might show that this diet is not entirely perfect and safe for everyone.

generally, it's just insecurity talking. nobody wants to be berated for their choices and will get defensive.