Why didn't the other Cyclopes help Polythemus?

The other Cyclopes did not help Polyphemus because they believed he was merely suffering from a divine punishment. When he called for assistance after Odysseus blinded him, they thought he was being tormented by the gods, as he claimed that "nobody" was attacking him. Additionally, the Cyclopes valued their independence and were not inclined to interfere in each other's affairs, especially when it involved a conflict with a powerful figure like Odysseus.