It hardly can.
Did you know that there were more greek soldiers fighting on Darius III's (50,000) side at the battle of Gaugamele/Arbil than on Alexander's side (38,000).
That's only if you take the inflated number of Darius' army seriously.
As for the OP, the Persian army was fairly good. They had some of the best cavalry around (probably only being outclassed by the Philip and Alexander's) and their infantry wasn't that bad, especially when mixed with greek hoplites. They had great archers and other light/range units. Where the Persians lacked was in tactical ability, which is understandable, considering they didn't necessarily have to have great innovative tactics for what they needed their army for.
It was more of "Philip and Alexander were just that good" than "The Persians were just that bad".