Why were there still so few Americans in California in 1845?

The big rush into California only came with the Gold Rush of 1848 - 1855, so several years later. Before that, California was only a territory as far removed from the 'civilized' parts of the USA as it was possible to be and plagued by almost continuous fighting by settlers against Mexican troops. Industrial activity - a big draw for immigrants in the east - was practically non-existent, and Californian harbors, such as they were, looked out on an empty ocean, because active shipping to the West coast would only start much later, after the opening op the Panama canal.