The reason was Vanaars too, could become part of civilized world. One fails to understand that though this fact automatically gets known, if myth of God, Goddess, and supernatural powers is removed from history, why are we making serious attempt to cover this society friendly Dharm established by Shri Ram?
Kaekai loved and adored Ram. HER Ram was on a mission, and HE needed her help for the sake of humanity and she decided to be on HIS side.
As it happened Bharat and Shatrughan had gone to their Mama’s(maternal uncle’s) place to spend some time, and Sage Vashisht informed king Dasrath of a highly auspicious muhurat, a few days ahead, which was not likely to repeat itself for a couple of years. He also told King Dasrath that the Muhurat was so strong and auspicious, that any work undertaken in it would give highly auspicious and positive result. King immediately decided that this muhurat would be used for coronation of Ram as Yuvraj. Celebrations started immediately.
Kaekai once again told him that she was all in favor of this coronation and is eagerly waiting to witness it. She also told Ram to stop thinking about this for the time being as his father was fairly old and may not be able to bear his exit to forest. Further, as a Queen and member of royal family her Dharm would not let her help him.
Kaekai was petrified. She bluntly told him that this was not possible. But Ram was not there to get a NO. He begged and cajoled her. She had the guts and she could do this, he told her. Kaekai told him if she accepted what Ram wanted her to do; she would be the most unpopular queen and a spoiler and would also be called a wicked step mother. Ram told her that he wanted to establish as dharm the need for single marriage, and what she would be doing would make his task a lot easy.