The true causes of gentrification have never been definitively determined. Many social scientists have speculated on the phenomenon, but there is no one answer to the factors that contribute to gentrification.
As cities developed over decades, the employment opportunities evolved -- for example, construction of a new factory or other labor-intensive industry opening up in a particular community would spur movement toward that new source of jobs -- and people moved accordingly. Middle-class whites, who were once concentrated in urban areas, moved out to suburbs as they became more affluent and modes of transportation evolved. The old neighborhoods in the city became occupied by lower-income families, who sought jobs in warehouses, factories, and other businesses that employed large numbers of low-paid workers, and over time those neighborhoods aged and fell into disrepair.
As urban neighborhoods grew depressed, the value of properties decreased accordingly, thereby making urban real estate inexpensive. Then came the process of gentrification, which often occurred at the expense of poor black families, thereby injecting a racial component into the equation. Those low-income African-American families were forced out of their neighborhoods by the increase in real estate values, and property taxes, that resulted from the purchase of land by developers hoping to tear down or renovate old structures and replace them with new, more expensive housing. Remaining African-American families were sometimes offered large sums of cash -- large by the standards of the lives being affected, anyway -- to sell their homes so that the developers could sell the land to more affluent whites.
There are many academic studies of gentrification, including one by the United States Centers for Disease Control, which studied the health implications of gentrification, but, in this educator's opinion, many of those studies over-complicate the subject. There is no question that multiple factors are involved, including the evolution of the economy from one based primarily on manufacturing to one based on services, the desire of categories of people to be closer to jobs or to better, safer schools, the desire to profit from the fluctuations on real estate prices, and so on. The bottom line is, however, that developers see opportunities in urban neighborhoods and, because the properties are relatively cheap and the potential profits from buying, developing, and selling or renting those properties for higher prices are tempting, the process of gentrification was going to occur.
This answer would not be complete if it did not include a pattern witnessed by this educator when he resided in Washington, D.C. -- a pattern that has been studied. That pattern involves the gentrification of run-down, primarily African-American neighborhoods in the District of Columbia by large numbers of gay and lesbian couples. A scholar at the Urban Institute in Washington, D.C. has studied the role homosexual communities have played in gentrifying neighborhoods in that city. The displacement of low-income African-Americans during this process was considerable, with those families forced to move to the suburbs.