Why were the athenians turn first to rebuilding the agora following the Persian war?

They turned first to rebuilding the defensive walls of the city for security, and adding to them with extra walls linking the city to its post. They also re-established the gods at the centre of their religious life - the Persians had wrecked them in retaliation for an earlier Athenian destruction of Persian gods at Sardis when Athens was involved in a Greek raid on the Persian provincial capital.

The agora, being a market paddock, did not need rebuilding.