srutva yuddhodyamam ramah
kurunam saha pandavaih
madhya-sthah prayayau kila
Lord Balarama then heard that the Kurus were preparing for war with the Pandavas. Being neutral, He departed on the pretext of going to bathe in holy places.
Both Duryodhana and Yudhisthira were dear to Lord Balarama, and so to avoid an awkward situation He departed. Furthermore, after killing the demon Viduratha, Lord Krsna put aside His weapons, but Lord Balarama still had to kill Romaharsana and Balvala to finish relieving the earth of her burden of demons.