A big problem in urban agriculture is space, specifically horizontal space
House lots in the city tend to be small, and apartments & condos smaller still. Even houses in the suburbs don't typically have forty acres of land to grow on or even a ten-acre hobby farm-type space.
My lot, for example, is 59 ft2, and my house sits on that. I estimate my yard space to be 1/4 of my lot size, not including setbacks. Since one person needs 4000 feet of garden space to grow enough vegetables for one year, there isn't enough space on the average urban lot for traditional row gardening.