Midwestern growing seasons only allow one crop per year.
Growing veggies is labor intensive and as things stand, it's easier (cheaper) to get migrant workers in central CA than in IA, IL etc-- although improved immigration laws could easily fix that. Eg- here in WI the dairy industry depends heavily on immigrant labor.
Once a pound of time ago, truck farms were common in the mid-west-- cf the Jane Wyman movie So Big about a widowed mother who made good growing veggies just outside Chicago.
The arrival of cheap refrigerated box cars made it more profitable to grow repeated crops each year in CA and ship them than to grow one crop here.
There's also the problem of fickle weather in the MW compared to CA. The Blackhawk War started when weather caused the Sauk's crop to fail and Blackhawk found it necessary to lead his group "off the reservation" to find food.
BTW- soy beans are not useless. They are used in an amazing array of industrial applications and, being legumes, serve to replenish the soil when they are alternated with corn (sustainability).