What if the miasma arrived on the scene after humans had developed civilization and industry? Maybe it was the effluent of some strange industry that got out of control, or was some natural toxin that was suddenly released into the air by mining or drilling? It was discovered that miasma would only rise to a certain altitude and so everyone suddenly had to relocate up to the mountains at a technology level of ~1850 or whatever. In my mind, this is a post-apocalyptic civilization, where the technology and knowledge to amend soil on shitty mountain dirt and build zeppelins already existed.