Why did the Southern Song Dynasty build its capital in Hangzhou?

Lin 'an was the capital of the Southern Song Dynasty for many reasons, mainly for two reasons:

On the one hand, nomads from everywhere take the initiative to attack, and Song Jun always defends passively, which was the long-term situation of the war at that time. Moreover, Jiankang is close to the front line and not as safe as Hangzhou in the rear. The water network in western Zhejiang is vertical and horizontal, which is not conducive to cavalry activities. Hangzhou has this natural barrier, which gives the rulers a sense of security.

On the other hand, since the Tang and Song Dynasties, Hangzhou has risen rapidly and become the "first state in the southeast" with abundant everything. This large-scale urban system and relatively strong material foundation are exactly what is necessary as a national capital.