We have the sugar lobby who paid researchers to create propaganda that fat was the culprit. We have companies incentivized to sell fad diets, we have every participant of the food industrial complex trying to push for their product to be consumed - and we have governments who are trying to keep the lobbyists happy and perhaps for the purpose of keeping the economy stable. They all aren't going to be promoting or funding research that will give results against their own interests.
Two videos I recommend everyone watch relating to diet:
Dr. Georgia Ede - 'Our Descent into Madness: Modern Diets and the Global Mental Health Crisis' - https://www.youtube.com/watch?v=TXlVfwJ6RQU
Dr. Jason Fung - 'Therapeutic Fasting - Solving the Two-Compartment Problem' - https://www.youtube.com/watch?v=tIuj-oMN-Fk