Why did Siraj-ud-Daulah lose the Battle of Plassey?

Before the battle, Robert Clive, the Governor of Bengal, won over some of the members of Siraj-ud-Daulah's court like his Commander-in-chief, Mir Jafar and Khadim Khan, who commanded the Nawab's troops. Due to this, major part of the Nawab's army, led by the traitors, did not take part in the fighting and the Nawab was defeated.