Wasn't the Trojan war fought precisely because Helen was not of Troy, but of Sparta? How come we call her "Helen of Troy."
She is actually also known as beautiful Helen, Helen of Argos, or Helen of Sparta, depending on the source - and not all sources associate Helen with Troy.
Euripides, Stesichorus, and Herodotus actually write Helen never went to Troy, but stayed in Egypt during the duration of the Trojan War.
The Trojan association is the most popular nowadays, primarily because of the general awareness of (the existence of) Homer's work...and Hollywood.