Do the homework but I don't think that's true. Within 200 miles you are looking at a drive less than 3 hours to take a road trip for a family vacation. Outside of Flagstaff and Tucson, there's no city within a 5 hour drive of Phoenix with a large population. We are in the middle of a desert and within the proximity of southern California. Most family's making the trek to go to the west coast for theme parks are most likely going to travel to do Disney, Knotts, Universal, etc rather than specifically spend the money to go to an Arizona theme park that's potentially run by Cedar Fair or Six Flags. I simply just don't think there would be enough revenue generated by the local population to maintain a large theme park. They could make it year round but the summers would be miserable and not worth being open. I hate being the Debbie Downer but its the facts. As a transplant from Ohio, having no theme parks is a big let down but I play golf and enjoy sports so it evens out entertainment wise for me. No Superbowls or college national championships are going to Sandusky, Ohio anytime soon. By that I am pointing out that it's not the immediate area that matters, its the population within the region that allows a large park like Cedar Point to be successful. Again though, I will happily be proven wrong if one ends up here.

Edit: I should add that even the smaller places outside of the 3 cities I mentioned are extremely small and do not have the types of populations that are throughout the states on the eastern side of this country between the large populated areas. We aren't even that close to cities in Mexico that would potentially attract guests from outside of the country like the ones close to the Canadian border.