My idea - When the food is nutritionally complete, and extremely bioavailable, providing everything you need, then you don't get bored with it. The brain doesn't force you to rotate it out in search of other nutrition.
I think this is like the opposite of Pica https://en.wikipedia.org/wiki/Pica_(disorder)
I don't have a good term for this, Food Satisfaction? Food Contentment?
The only time I have to mix it up is when I have house guests, and I need to be a good host.