When the city of Phoenix was younger, back before the common use of cars, people had to either ride a horse, take a trolley car or walk. It's still true in many parts of the world where cars are uncommon. And because of that, the poorer people in Phoenix were condemned to areas away from convenient public transportation. And mostly, people without much money to spend had to walk. And walking up and down hills is something that most people with any sense will avoid in the normal course of their life. Sure, it's fine for hiking on the weekend, but not for carrying groceries, etc.
So the sloped areas of Phoenix, away from public transportation, like Sunnyslope, are where the poorer people lived. The wealthier people lived where it was easy to get around, on the flat land. It's all backwards today, as wealthier people have for many decades sought out hilly and mountainous places to build their homes, and enjoy the view.
But they get there with cars.