Fires spreading that fast through a ship that size means the fire must have been spreading through some of the mechanical ducts of the ship, in close proximity to the plasma lines, and threatening to reach weapons storage, engineering in short order.
Obviously it didn't, but staying aboard would have risked the entire crew on the that gamble, they were under fire and didn't have time to get the panicking crew to fire control stations or environmental control.
They probably assumed that either the ship had been destroyed, the fire consumed it and all but destroyed it or it had been captured, either way there was no point returning.