A city doesn't necessary need to have a mall. All other cities in the game all contain at least a Pokemon centre and a pokemart. Mauville City has that and is almost the exact center of Hoenn. It is pretty much the main attraction. Also, many people come, sell, and visit Mauville City. The mall is just a main representation of that distinctive city. Hoenn is based on Kyushu, Japan and since Japan is such a busy place where lots of people shop, visit etc., the creators probably wanted to base the big idea on that.
Hope I helped! :)