it could be entirely possible that they'd get reinforcements; after the Carneia festival ends, the Spartans could send the entirety of their military to Thermopylae. it was because of that that only Leonidas and the 300 (as well as a much larger number of Phoceans, including helots)
How long would that take? I'm unsure of the length of the Karneia. And the Spartans may have assumed that Leonidas and them were already dead, given the length of time it would take to get word back.
Of course, if you delayed the Persians invasion by quite some time (spotty entrails or two-headed black cats or something) you might avoid the Karneia altogether and Sparta could move with her whole strength.