Why didn't Hanzhong pass Xi 'an before?

It's hard to build. According to the query encyclopedia, the railway in Hanzhong was previously impassable from Xi, because there were many mountains between the two places, limited equipment and difficult construction. Hanzhong, referred to as "Han" for short, is also known as the city of mountains and rivers, a small Jiangnan in the northwest, and was called Nanzheng, Xingyuan, Liangzhou and Tianhan in ancient times. It is a prefecture-level city under the jurisdiction of Shaanxi Province.