West Coast swing is a direct descendent of the Lindy Hop, and probably developed in more than one place at more than one time. West Coast Swing originated from an earlier dance known as the Savoy Style Lindy, which was done at the Savoy Ballroom in New York in the early 1930's. Although WCS was not invented by, it was indirectly spawned by a man whose name was Dean Collins, who also danced at the Savoy while living in New York.
There are examples of Lindy being slotted, though not in the strict style of today's West Coast Swing, long before the emergence of the separate style.
There are many origin legends for West Coast Swing:
dancers and choreographers slotting the dance for easier filming for movies and television;
crowded dance floors and group tensions forcing the slotted style to accommodate more dancers and keep exuberant back movement contact from erupting into fights;
slower R&B music becoming popular allowing increased footwork and languid body movements as musical expression.
Most likely, there is truth in each of these. Dance studios helped to formalize the West Coast style by teaching it as a separate dance from Lindy.
One story is that Arthur Murray culled out the 6-count pattern from Lindy and dubbed it 'East Coast Swing', and dubbed the new slotted style 'West Coast Swing', thus having 3 different dances to teach instead of one.
West Coast was also originally called 'Western Swing', but the '- Coast' was added to differentiate it from Country & Western styles. Swing dancing is our national folk dance. It, like the music that inspired it, Jazz, is uniquely American. It is urban, rather than rural. As popular music changes, so too does the dance.
More on the history of WCS:
Sonny Watson, has the history of West Coast Swing, here, on his website :
Pretty in-depth description: