Why did the Greeks choose Thermopylae as a defensive position?

The Greeks chose the narrow pass of Thermopylae as a defensive position during the Greco-Persian Wars primarily to neutralize the overwhelming numerical superiority of the Persian army led by Xerxes I.1

The strategic advantages of the location included:

• Constricted Battlefield: The pass was situated between a steep hillside and the sea, creating a narrow front only a few dozen yards wide. This geography prevented the Persians from deploying their vast numbers effectively, forcing them to engage the smaller Greek force on equal terms.1
• Defensive Chokepoint: By holding this narrow line, the Greek forces under Leonidas were able to block the southward advance of the Persian army into Attica and Boeotia while the Greek fleet operated in the nearby straits of Artemisium.21

This position allowed the Greeks to inflict heavy casualties on the Persian forces for two days before the position was compromised by a flanking maneuver through a mountain path.1