The neighboring states of California and basically everything besides Texas that is west of the Mississippi is pretty low population. How, and why, did California in specific become so high in population and economy?
Carey McWilliams in "Southern California: an Island on the Land" said of Southern California that the population grew not so much in booms but in one long series of boom punctuated by a number of explosions. The same could be applied to the state on the whole, the only difficulty is where to begin one's historical reckoning.
Prior to Spanish colonization under the Mission system beginning in the early 18th century in what they then termed Alta California, the area is projected to have had one of the highest density native populations throughout what is now the U.S./north of modern Mexico. I have stated this specifically to call out your use of the word "develop" which I take some issue with, but anyway.
Through the Spanish (and Russian) periods population didn't grow greatly, but during the Mexican period (1821-1848/9) several interesting things began to happen:
There was an increase in trading along the coasts, with several of the merchants marrying into wealthy local Californio families. These marriages were prized by the Californios, because of the perceived social benefits of producing a "whiter" family line.
At the same time, following the arrival of the mountain man Jedediah Smith into California from an overland route in the mid-late 1820s, a growing number of other mountain men began to illegally enter California, then still under control of Mexico. Because Mexico was cash and resource strapped at the time, having just waged a battle of independence and struggling to keep its center under control never mind its peripheral border zones, this was tolerated at first, even somewhat benevolently as a means of building up population along relatively underpopulated borders. Until Texas broke off in 1836, when suddenly it was frowned upon, but still not exactly stoppable. This situation continued, escalating, through the 1840s, when the mountain men began leading whole family groups in to settle California, which in turn eventually led to the Bear Flag revolt in 1846 and then coalesced into Fremont's essentially illegal usurpation of California from Mexico, for the United States. But I (bitterly) digress.
(edited to add sources to first of three posts)
Sources:
College level courses in the history of Los Angeles, California, Indigenous History of Colonial Mexico, and North American Indigenous History.
Supported by these readings:
Avila, Eric, Popular Culture in the Age of White Flight: Fear and Fantasy in Suburban Los Angeles
Davis, Mike, City of Quartz
Deverell, William, "Whitewashed Adobe: the Rise of Los Angeles and the Remaking of it's Mexican Past*
McWilliams, Carey, Southern California: an Island on Land
Rice, The Elusive Eden: a History of California
Wilson, James, The Earth Shall Weep: a History of Native America
Aaaaand quite a few primary source readings on 18th and 19th century California Mission history.