A significant reason for American wealth in the 1920s was the government's encouragement to raise wages, as well as drastically cutting the income tax on the highest earners, thereby freeing capital for re-investment into the market. With the laws and economy favoring business, and radical elements shamed or legislated out of relevance, the circumstances were perfect for growth. While the richest elements remained rich, and were largely the only ones to directly benefit from a booming stock market, the middle class also saw real growth, such as in the beginning of American car, radio and other forms of popular culture, made possible by mass production and mass access to that production.
On the contrary, rural America suffered during this time as its jobs disappeared or relocated to factories, and its population shriveled. Rural elements and rural culture were rapidly overtaken or outdated by the shift toward commercialized mass consumption. This became evident during the Great Depression, when a considerable number of rural and agricultural residents, already clinging tenuously to a shrinking way of life, were displaced.
The 1920s also saw the continuation of internal struggles, such as the enactment of Prohibition and combating of organized crime. In short, America had plenty of things going on at home; we didn't resemble the "international peacekeeper" that we became after World War II. Politically, the United States was extremely reluctant to engage in any foreign affairs after the fiasco of World War I. Perhaps the best example of this is the League of Nations; despite being conceived and formulated by the United States, the US was never a member of the League, which was ultimately considered ineffective. This sentiment also led to increased resistance to immigration and immigrants themselves.
In short, the 1920s were rapidly and drastically redefining what it meant to be American.
Why did some embrace the era's changes, while others resisted them?
Those who accepted and welcomed the era's changes were largely those who stood to profit from them; the business owners, most of the middle class, conservative politicians, manufacturers and artisans. The increased market and ease of access to that market was a boon for anyone with the ability to create something and the capital to do so, and as long as you weren't a political radical. There was much more to see and do in the cities as well.
Those who resisted the changes were those who stood to lose power or comfort, such as rural workers, immigrants, the poor, radicals or, in the case of Prohibition, those who enjoyed alcohol. Resistance came in various forms, both physical, social and political, and was not always a dichotomy of poor vs. rich; for example, Prohibition came about by the discomfort felt by those in power at the increasing personal freedoms Americans were enjoying, and their concern that this would lead to immorality.