Because the supporting Persian fleet had left the army and sailed for Athens; and the Greek soldiers (the heavily armed and tactically well organized and professional Hoplites) did a better job manoeuvering and fighting than the lightly armed and more loosely organized Persian soldiers.
Tactically, the Athenians drew the Persians into a trap by withdrawing in the center while advancing on the flanks, thus enveloping the Persians. The Athenians were able to cut down many of the fleeing Persians while they were attempting to reembark on the remaining ships.
Tactically, the Athenians drew the Persians into a trap by withdrawing in the center while advancing on the flanks, thus enveloping the Persians. The Athenians were able to cut down many of the fleeing Persians while they were attempting to reembark on the remaining ships.
