To live longer.
To paraphrase from a book by Alan Lightman, ‘Einstein’s Dreams’
To slow the passage of time, and live longer lives than others, the wealthy lived deep in the gravity well and the poor lived high atop the mountains and in orbit around the planet where time passed faster. The amount of difference in the relative length of life was barely seconds but it was the idea of it, of losing even a moment of life to forces outside their control, that resulted in the practice.
For same reason, people wanting to live longer. But it was because the air was polluted by particulate matter that slowly poisoned the inhaler. The city where this was the practice, of living as close to the ground as possible, the flora helped cleansed the particulates and the shape of the valley -- similar to Denver -- caused steady airflows that kept the air near the surface fresh.
But higher up, the brown cloud of particulate poison hangs over the city -- like Denver and its famous brown cloud -- trapped by a seemingly near-constant inversion layer. Some days, in winter, the temperature inversion clears and the particulates are blown out of the valley, but it starts accumulating again.
Now, in the future, they built giant domes and processed the air, removing the danger, but the status of living close to the ground, demonstrating one's virtuousness and prosperity, is deeply embedded in the society.
They use strange sounding phrases like 'may you sleep in the mud' 'dirty shoes -- as tidings of encouragement and celebration.