Why did Pharaoh refuse to let the Israelites go?
Pharaoh refused to let the Israelites go primarily for economic reasons: he could not afford to lose the massive slave labor force, believing that releasing them would ruin the nation. Although God is sovereign and Pharaoh had free will to comply, he chose to keep Israel enslaved to protect Egypt's economy.
Les explains that this is a tough passage, but God is sovereign and absolute in power; we are in no position to argue what He does or why. Yet God is righteous and gives every human being free will. Pharaoh could have let the Jews go when confronted, but he refused because of economics: he could not lose the slave labor without ruining the nation, and it did ruin Egypt when they left. Les connects this to the potter and clay passage, where the potter has power over the clay to make vessels of honor and dishonor, showing God's sovereignty and long-suffering while Pharaoh exercised his own will.