The answer is as follows.
The emperors of the Southern Song Dynasty had bitter enmity with the Jin State. Because the Northern Song Dynasty was destroyed by the Jin Dynasty in Jingkang.
The Southern Song had allied with the Jin to destroy the Liao. Later, the Jin treacherously allied with the Southern Song to destroy the Liao and then began to attack the Southern Song. The Southern Song could no longer cooperate with the Jin.
The second is that the strength of Mongolia is not the Southern Song and Jin can be destroyed.