Why couldn't the Han dynasty expand into the southwest or far northwest?

The Han dynasty faced significant challenges in expanding into the southwest and far northwest due to geographical barriers, such as mountains and deserts, which made military campaigns difficult. Additionally, these regions were inhabited by various non-Han ethnic groups, including the Qiang and Xiongnu, who resisted Chinese control. Political instability, resource allocation, and the need to maintain order in more central territories also constrained Han expansion efforts in these areas.