With California, the answer is relatively easy. The indigenous people were numerous in the lush environments that would become the state, but they sufferer from disease and from harsh measures by both the Spanish speaking and the subsequent Anglo settlers. The speakers of Spanish were fairly numerous, but their population was limited by the fact that it was at the far reach of the center of Mexican population and political control much farther south. After the Mexican-American War (1846-1848), the territory would see an increase in Anglo settlers who had begun to arrive in the years before.

California was, however, a long way from the Eastern centers of Anglo population, so the inspiration to overcome the obstacles to get to California was not too great until the discovery of gold in the Sierra foothills in 1848. This caused the famed Gold Rush of 1849, which attracted tens of thousands of fortune seekers from throughout the world - but especially from the Eastern states. This propelled California into statehood in 1850 - one of the fastest transitions from territory to state in American history.

Texas is another matter. In this case, we have a place that was under control of Mexico but which was attracting "spill-over Anglo settlers." This steady flow of Anglos consisted of people who were seeking good land for settlement and who were finding the United States increasingly "locked up." In the case of Texas, the perceived opportunity was considerably closer than the remote Pacific shore of California, so it presented an obvious opportunity, and Anglos responded. With Texas statehood and then its integration into the United States, the inspiration for Anglo settlement increased, and proximity opened the door for it to happen.

I'm not as knowledgeable about California as I am Texas, so I will only be answering the portion of this question regarding Texas.

When Texas was apart of Mexico, it was very sparsely populated with some Tejanos, Anglo-settlers, as well as native tribes. After the Texas Revolution, there were concerns over the new borders between Texas and Mexico. This was one of the factors that eventually led to the Mexican-American War 1846-1848.

After Texas was annexed into the U.S. as the 28th state, land in Texas was extremely cheap compared to other places in the country. Eventually cattle would become a large part of the economy of Texas and several cattle trails were established, notable the Goodnight-Loving trail. This industry had much less discrimination compared to other industries due to the danger involved. As the industry later slowed due to the development of barbed wire, other industries popped up to replace it.

During the 1880s, the coal industry started to sprout in Texas, with 1884 being the first year of commercialization. In general, the coal industry in Texas thrived up until WW1, where there was a dip in production. Between the first and second world wars production continued, but post World War 2, production was nearly non-existent. This would pick back up later in the 1970s and today Texas is in the top-10 coal producing states in the country.

As well as coal, by 1860 there were over 200 sawmills operating in the state and the lumber industry became a vital source of work for many in Texas. In fact, by 1907 Texas was the third largest lumber producer in the country.

All of the clearing of forest lands led to those lands being surveyed for oil, and the discovery of the black gold in 1901 which lead to the Texas Oil Boom. At the time, the petroleum reserves in Texas were the largest known reserves in the world. This find was unprecedented and lead to the rapid development and industrialization of the regions. This boom age continued well into the 1940s, and to this day oil is a vital part of the industry in Texas.

In conclusion, when Texas belonged to Mexico & Spain there was not nearly as much commercial and economic value in it as there was later when it was integrated into the US. The discovery of valuable resources combined with cheap affordable land lead to population booms in Texas several different times over the course of nearly a century.

EDIT: Wanted to reword my final paragraph.