For the same reason debates about which band is best in a given category usually end in a screaming match: because (i) to quote Nietzsche, God is dead, and (ii) people see the foods they eat as an expression of their identity and thus become ego-invested in certain foods. Like Paarlberg (2013) notes on pp. 182-183 in the second edition of his Food Politics (a must-read for anyone interested in food and agricultural policy):
Groups in society have always sought solidarity through the foods they eat, or the foods they agree not to eat. Within most religious traditions, patterns of food consumption are carefully regulated. …
In today’s less religious world, we should not be surprised to see the emergence of new food rules to express solidarity around secular values. The new rules that emerge (organic, local, or slow) may be attractive or practical only for relatively small subcategories of citizens, or perhaps only for a small part of the diet of those citizens, but the exclusivity and difficulty of the rule can be part of its attraction. The goal is to express through the diets we adopt a solidarity with others who share our identity, our values, or our particular life circumstances. The scientific for these modern food rules may at times be weak, but the social value can nonetheless be strong.