Traditional kibble is made with a manufacturing process called "hot extrusion," which requires a high starch content to bind the pieces together. Historically, dog food manufacturers used the cheapest sources of starch available, which in the US in recent decades has been corn, wheat and soy — due to farm subsidies, but that's another story.
There actually isn't anything inherently bad about these ingredients (with the possible exception of soy due to its potential role in hormone interference), but there isn't anything particularly good about them, either. Again, the primary reason they became a staple in dog food is merely because they're very cheap — not because they have any special benefits for nutrition and taste.
That is why you'll find higher-end dog foods tend to avoid these ingredients, including Sundays, because we want to use the best possible ingredients — that offer special benefits for nutrition and taste, not just the cheapest source of calories.