Why did the Athenians and Plataeans win the Battle of Marathon?

Strategy and Site

The choice of Marathon proved suitable for the Athenians, as its strategic position, on the heights commanding both roads to Athens, allowed them to wait in safety until the right moment to attack.

Miltiades grasped the right time to engage the Persians, when he was informed that the Persian cavalry was absent. The fact that the expert Persian cavalry took no part in the battle was one of the significant reasons for the Greek victory

Leadership

Callimachus, as an elected commander-in-chief, listened to the advice of Miltiades, who had first hand knowledge and experience of Persian fighting methods and arms.

The political leadership of Callimachus and Miltiades in convincing the Athenian Assembly that they should send an army to Marathon prevented Miltiades' enemies from giving aid to the Persians 

Difficulty of Escape

Miltiades knew the Persians would position their best troops in the centre, and the disposition of his infantry allowed the wings to encircle the stronger Persian centre. The charge of the Greek Hoplites created surprise and confusion among the Persians (for this was the first time in history that the Greeks had charged into battle), as well as allowing the Greeks to get close to the Persian bowmen before the latter could release their barrage of arrows

The Persians were confined between the sea and the hills and their only chances of escape were to flee to the north (where many perished on the edge of the great marsh) or to reach their ships, which were standing offshore

Skill, discipline and arms of the Athenian and Plataean Hoplites

Although only citizen soldiers, the Greek hoplites were far more disciplined than their Persian counterparts and also better protected, with their bronze-visored helmets, solid bronze breastplates, shields and javelins

The Persians were generally lightly clad, with wicker shields and bows and arrows, although sometimes Persians had body armour of scales sewn to leather vests

Fear of Sparta's arrival

The possibility of the Spartans marching to Marathon had a real influence on the campaign by forcing the Persians and Athenians to hurry up their operations

The Greek's defense of their freedom

Freedom to rule themselves in their own way, without a tyrant or an 'oriental barbarian' overlord, was the motivating factor behind the young democracy's stand. In defending their homeland, the Athenians proved more solid and united than the opposition had hoped