Herodotus suggests (although Plutarch disagrees) that the Theban army was forced to fight because of Thebes previous desire to capitulate to Xerxes. True or not, Herodotus would have influenced the thoughts of people for thousands of years. Furthermore after Thermopylae, Thebes DID side with the Persians, Sparta did not.

The Thespians are unjustly unsung. Their army was essentially their entire male population, whereas the Spartans had thousands of hoplites left in Sparta. The Thespians fought in order to protect their people from being killed by the Persians and allow them to evacuate northern Greece.

As to why the Spartan sacrifice is more famous? Sparta and Athens where enormous compared to most greek poleis, and massively powerful in the area, most other cities at this time were tiny. The Spartan political and social model made warfare their specialty, in essence they are famous for fighting.

Also later on in the war when Sparta commited a proper army (they had been busy at festivals during thermopylae) they led the war against Persia on land. Retroactively adding more prestige, Thespae continued to be a minor polis and Thebes did not become a major player until after Herodotus and Thucydides had written their histories.

Add to that hundreds of years of Romanticizing, the Spartans are more famous. Although there is a monument to the Thespians next to the Spartan one now.