Taking a casual survey of the lay of the land, as it were, one might notice that there are certain fashion trends that develop regionally based on the needs of the people in that location, and these trends sometimes spread to other areas, although the fashion rarely lasts for longer than a season elsewhere. Take the layered clothes look. This mostly developed in the San Francisco Bay Area and the Pacific Northwest. The people there were dealing with varied weather and needed to be able to put on or remove layers over the course of the day just to be comfortable.
There is another way that fashion tends to spread though, and often it doesn't involve seasonal fashion trends, but rather cultural ideas. As in the case of western wear, for example, people have a concept of the types of people who wear what they would sweepingly deem as cowboy apparel. They also have the historical concept of the cowboy, cattle drives, frontier towns and so on that inform their mental picture of where western wear came from. Because there is no longer a specific location in the country where western apparel is routinely worn for work and play, there is a certain stability to most people's concept of this style of dress.
The constancy of western wear means that it has had the opportunity to be worn- cowboy boots especially- by people all over the country, who view this as simply a style of dress that they can adorn at will (whether their purposes be fashion or function), borrow from to combine with other fashions, and utilize aspects of to create a certain look. Certainly, people carry this style of dress with them after living in areas where western wear is the norm, but you do also see people adopting it as they move closer in proximity to cowboy territory, or even just as a result of enjoying country music. The Nashville country music scene has done a great deal towards bringing western fashions to the East Coast. It becomes a part of the identity of the fan- even if they've never ridden a horse, mucked a stall, or seen any kind of livestock close up and personal before.
All said and done, western wear as a style of clothing is one that will likely remain on the fashion menu, both because it continues to be worn by people of that culture for work and for fashion, and because it has a place in the cultural history of this country.
A cultural and historical look at the role of
western wear in America will tell you that it is an aspect of fashion that is consistent, whether it is worn as a whole or just in elemental pieces. It is more stable than the passing trends because of it's solid connection to American culture and history.