Because much of people's preference for certain types of food are acquired and not necessary (or even beneficial). During times of poverty, there are often pressures to change preferences, leading to a much higher rate of innovation. (Special case of general mechanism in creative destruction / evolution)

Once a preference is established, it can stay for a long time because there's a physiological tendency to like food you've eaten (without ill effects) before. There are few social forces strong enough to override that.