Why is Ruoxi in "Walking on Thin Ice" only a side Fujin?

Because Ruoxi's family background is not prominent enough.

Walking on thin ice, Zhong Ruoxi first came to the Qing Dynasty and lived in Mei's mansion, where she met her lifelong friend Mei. She and her tenth brother are childhood friends, and their personalities are very congenial. In particular, Brother Ten showed extraordinary love for Ruoxi.

Ruoxi also specially organized a birthday party for Brother Ten, so everyone thought Ruoxi liked Brother Ten. However, during the festival, Kangxi also knew that Ruoxi's name in Beijing was "Thirteen Sisters Desperate" and understood Ruoxi's ambiguous relationship with May 10th, but he still betrothed Mingyue Gege to May 10th in public as a personal blessing.

Brother Shi has reached marriageable age, and Kangxi has already considered a good candidate for him. This person can only be Mingyugege, because Ruoxi's identity is not as high as Mingyugege. Ruoxi's sister Lan Ruo can only be a side Fujin in May.

If the sunrise fell in love with grandpa four:

Ruoxi is a person who has experienced a lot with grandpa four. In times of difficulty, their hearts can lean together, but when things get better, they can't get any closer. Ruoxi lives in the memory of only her and grandpa four, and he has always been her love.