Maharaja Dasarath, although he was a great devotee of the Lord, but because he was a Ksatriya king, keeping one’s promise is inevitable for him. He preferred to banish Ramacandra on the request of his wife on the principle of keeping his promise. In higher states of spiritual life, one can break even promises also, but they are devotees of comparative merit. In the case of Vasudeva, we find that he was spiritually more advanced than Maharaja Dasarath. Vasudeva was also in agreement with Kamsa that he would deliver all his sons to his hand, as soon as the child is born. But in the case of Krishna he broke his promise. The point is that Krishna appeared as the full fledged Personality of Godhead. But Ramacandra appeared as an ideal king. Therefore in the Lila of Ramacandra, principles of morality and ethics as they are to be followed by ideal king and ruler was followed. In the same sense, He banished Sita in order to prove Himself as an ideal king who wanted to make His subjects always happy. The whole program was on the basis of an ideal king. But in the case of Lord Krishna, He played as full independent Supreme Personality of Godhead. Apparently, therefore he transgressed so many moral and ethical principles. These comparative studies on the life of Krishna and Ramacandra is very intricate, but the basic principle is that Ramacandra appeared as an ideal king and Krishna appeared as the Supreme Personality of Godhead, although there is no difference between the two.
Ref >> Srila Prabhupada Vani.