Why did Arjuna, who was so close to God, get a degraded life in his next birth?

Mere closeness to God does not give the top result. The son of a professor may be a student in his class and this does not mean that the son should pass with a first class. Even the son of Lord Krishna, called Samba, mocked some sages and became responsible for the genocide of the entire Yadava family. The result is based on the firmness in faith and intensity in devotion, which are responsible for the excellent practical service that gives the final fruit. In fact, closeness is a negative factor. Closeness brings dilution of care and faith. When Rukmini drank the same hot milk, Krishna did not become red since Rukmini was very close to Him. In the case of Radha, who was far, the opposite happened. Of course, the topmost devotees are exceptional. Lakshmana and Hanuman were always close to Rama and had the same devotion of Radha. Closeness leads to repulsion and this applies to general lot of humanity. Arjuna belonged only to this general lot and hence, was called as ‘Nara’ or a general human being. In fact, Arjuna represented a general human being only to show how the human being errs by neglecting the close human form of God.