Queens did travel - they just didn't show Sunaina. (In the previous serial, they did show Sunaina, in the aftermath of the coronation, when Rama and Sita did charan-sparsh to Sunaina, Kaikeyi, Sumitra & Kaushalya.)
I think it was just an oversight. I was more curious about why UMS were with their mothers-in-law, rather than their hubbies?