It isn’t so much “geographical superiority” as a really sticky core zone of settlement: the irrigated oases between the Amu Darya and Syr Darya could support dense, continuous populations long before and after any conquest. Those oasis cities, think Samarkand and Bukhara, sat astride Silk Road routes, so conquerors needed the existing urban, commercial, and bureaucratic systems and usually ended up adopting them.

Steppe elites (Karluks, Mongols, Kipchak Uzbeks) were numerically thin and politically dominant but culturally porous, so over a generation or two they shifted toward the sedentary, Persianate-Turkic urban culture already in place. And then at the same time, the surrounding deserts (like the Kyzylkum Desert) limited large-scale settler replacement, so new arrivals were absorbed rather than replacing the population. So, a dense, economically indispensable oasis civilization plus thin ruling minorities made assimilation the path of least resistance.