Uzbekistan is located in the fertile valley between the rivers Amu-Darya and Syr-Darya, and its territory includes most of the ancient cities in the area like Bukhara and Samarkand. It also includes the majority of the fertile Ferghana Valley. Simply put, Uzbekistan's territory includes the core of most older states that existed in the region. Kazakhstan is mostly steppe, Turkmenistan is mostly desert, and Kyrgyzstan and Tajikistan are mostly mountains. So yes, I'd say geography plays the biggest part.