When Amish launched Rama, the first book of Ramayana Series, he told that he would be writing this series in a multilinear way: the lives of Ram, Sita and Ravana will be described in three different books and then be merged together on a single most important event of their life: Kidnapping of Sita.
Rama was good: plot line was being set up and Sita was better: even if some things were repetitive, story-line was opening to a next level and writing was also good.
So, Ravana is fighting because of His love for vedvati, Rama is fighting because of His love for Sita, Kumbhakarna is fighting for his love for Ravana, Vishwamitra and Vasistha are fighting for their long-lost love.
I am not sure what actually went wrong with this book, maybe Amish was struggling with his personal life and he just wanted to get this completed because it was the most dark book he ever wrote.