In many parts of the Earth, e.g. in Europe, maize a.k.a. corn and wheat are much cheaper energy sources than anything else and wheat and corn are about as cheap.
Wheat may be preferable to corn only because it has a double amount of proteins, so it can also be used to provide a big part of the daily intake in proteins, if it is supplemented with at least another source of proteins that is rich in lysine. However, wheat is more suspicious from the point of view of its health effects, so when the absolute minimum cost for the daily food is not the target, corn might be the best energy source in food.
The bulk of the food that is needed daily is required for providing energy and corn can do that at a minimal cost and it can be cooked into tasty food.
The rest of the food must provide proteins, essential fatty acids, vitamins and minerals, but much less amounts are needed for that.
So corn is perfect nutritionally for its right purpose. Obviously it cannot be the only kind of food that is eaten.
Except for energy (i.e. starch), most cereals do not contain anything in sufficient amounts to be an adequate source of it. Wheat is an exception by having a useful amount of protein, which is however harder to digest than most other proteins and the undigested fragments may have undesirable effects. Only oats has a higher protein content than wheat, but oats is much more expensive, so it is not really competitive from this point of view.
Therefore it makes no sense to claim that corn is worse nutritionally than other cereals, because it is better than most (except possibly rice, which cannot be cultivated in the places where corn is grown) at the only thing for which cereals are really good: providing energy.