Well, one could also say that the soldiers of a culture very heavily based on warfare (Sparta) had a superior position and were really determined professionals.
The Persians had their Immortals, but they must have made wrong decisions, otherwise the Spartans would not have held the pass for so long.
After they fell in their back, the superior numbers and tactical advantage shifted to the Persians, killing all or nearly all defenders, giving the battle this grandeur that made it memorable over a thousand of years.
This last stand scenarios are often portrayed in a heroic way, just imagine yourself standing there at the opening of the pass, and then notice some Persians falling into your back... now this is a real feeling of hopelessness, you know that you will die for sure.