Why did the Senate send Pompey to Spain to fight Sertorius?

The Roman Senate did not initially choose to send Pompey to Spain; rather, Pompey maneuvered to secure the assignment himself. After helping to suppress the attempted revolution of Marcus Lepidus in 78 BCE, Pompey refused to disband his army. He used the threat of his military force to pressure the Senate into granting him proconsular power to join Quintus Caecilius Metellus Pius in Spain.1

The objective of this campaign was to reestablish the Senate's control over the region, which had become the stronghold of the Marian leader Quintus Sertorius. Sertorius had successfully defied the Senate for years, ruling over most of Spain and defeating previous Sullan governors. By 77 BCE, the Senate faced a persistent threat from this independent power base, and Pompey leveraged his military leverage to take command of the effort to defeat him.21