Why did Pompey replace Lucullus in the war against Mithradates VI?

Pompey the Great was appointed to replace Lucius Licinius Lucullus in the war against Mithradates VI primarily due to political maneuvering in Rome and the military difficulties Lucullus faced toward the end of his command.12

While Lucullus had achieved significant early successes against Mithradates and his ally Tigranes II, his position was undermined by several factors:

• Military Mutinies: Between 68 and 67 BCE, Lucullus’s troops staged three mutinies, which forced him to curtail his military operations and allowed Mithradates to recover much of his lost territory.2
• Political Opposition: In Rome, Lucullus had earned the enmity of Roman businessmen due to his administrative policies in Asia, which limited the interest rates they could charge local cities.2
• Legislative Action: Taking advantage of Lucullus’s weakened standing, his political enemies in Rome successfully passed the Lex Manilia in 66 BCE. This bill stripped Lucullus of his command and transferred it to Pompey, granting him full authority to conduct the war and reorganize the Roman East.12