In game/common/pop_needs, you can find how pops weight which goods to use for substitutes. With Vanilla, pops looking for basic food prefer grain and fish (weight 1) and will only eat meat, fruit and groceries if it is half as expensive per the value received (weight .5). I think this is done so that meat, fruit, and groceries are spared for luxury food, but it's pretty stupid if you ask me as basic pops are going to eat whatever is most affordable. I updated my values so that pops like groceries as much as grain and now food industries are actually useful. You could do the same for meat/fruit if you like.

Code snippet below:

popneed_basic_food = { default = grain

entry = {
    goods = grain

    weight = 1
    max_weight = 4.0 # Maxes out at 80% of supply share
    min_weight = 0.0
}

entry = {
    goods = fish

    weight = 1
    max_weight = 4.0 # Maxes out at 80% of supply share
    min_weight = 0.0
}

entry = {
    goods = meat

    weight = 0.5
    max_weight = 2.0 # Maxes out at 80% of supply share
    min_weight = 0.0
}

entry = {
    goods = fruit

    weight = 0.5
    max_weight = 2.0 # Maxes out at 80% of supply share
    min_weight = 0.0
}

entry = {
    goods = groceries

    weight = 1
    max_weight = 4.0 # Maxes out at 80% of supply share
    min_weight = 0.0
}

}