Answer:
It broke off on its own against Mexico in the Mexican-American war, so the American government wanted to award it with statehood. Unlike other states, which had to be taken manually by the US, California seceded by itself like Texas.
Another reason is the Gold Rush, when many thousands flocked over to California, the Federal Governent was pressured to set up a civil government.
The ensuing gold rush which lasted 2-3 years sped California's statehood.
