These wealthy communities tend to employ those of poorer communities, making it increasingly more difficult for the underprivileged workers to develop further than their employers, the wealthy, regulate. The gap between the different economic statuses then become painfully obvious. The separate communities develop and progress further and further apart (metaphorically, not physically) and frequently very shabby, lower-class communities will form adjacent to ritzy neighborhoods the longer the one-way progression of the privileged occurs.
However, to understand how the wealthy individuals pick their prime spots to start these ever-flourishing communities, one must take into account various other factors, primarily location. Geography has as much to do with the separation of development as money does. Naturally, the upper-class privileged people will pick prime spots for their homes (or in many cases, vacation homes) and will collectively claim their stakes in the land and surrounding areas. Outrageous prices for these wealthy-controlled areas prevent anyone but the wealthy from developing on said land, and thus the one-way progression I've mentioned earlier begins to develop.
On certain occasions, physical boundaries, such as the frequently mentioned Winter Park railroad, will also be used to indicate the beginning of a rich neighborhood and the end of a working-class neighborhood, or even as a boundary for the underprivileged who are pushed out of their original locations by being bought off their land. However, it is painstakingly obvious that those pushed off their property are usually non-whites. To understand this, one needs to flip deep into the history books. The superiority of the white man felt for countless years drove slave trade and underpaid immigrant workers. Although many have put a great effort to relieve the injustice and create racial equality, that superiority was at one point socially acceptable. That acceptability allowed whites to take advantage of minorities in business and social situations and naturally gave the best options to the white man first.
With that being said, once at the top, it is hard to be dragged down. The top, after all, controls everything, including the things that keep them at the top. Priority is given to the highest bidder.