Partly you are confusing eras, but also sections of society.

The 1790s was indeed a time of "wigs and thights", for those few who were on top of the social ladder. The wild west of cowboy fame is largely a 1860s-1890s post civil war phenomenon, so we are talking several generations between the two.

Finally you are comparing an established "urban" society with frontier society. The "wigs and tights" were very much a status statement, modelled on European norms. Such clothes (although wigs were beginning to go out of fashion even then) were worn by the upper crust as a means of representing their status and station. You have to think of it as formal wear, to be put on for certain occasions. Everyday wear would often be more practical, even for the wealthy and powerful.

Frontier or working dress would have been much more practical, not to mention cheap/durable. That goes for both the pioneers and settlers of the 1790s, and the cowboys of the 1880s.

That said, there are many similarities between the two. First there is a larger degree of similarity between shirts, coats, vests and hats worn as everyday wear in the 1790s and the 1880s than you might think. Remember that the John Wayne cowboy "uniform" as recognised today is to a large extent a Hollywood creation, just as the myth that all revolution era New Englanders wore tights is. Secondly, fashion and mode of dress across all eras and societies is largely a question of identification. Your clothes marks you as a member of a group (or symbolise your aspirations). Thus we tend to try to use the same clothes as people we identify with. That means that even if you for some strange reason wore tights in the 1880s, you would probably swap them for trousers had you moved to a western frontierstown, just because that's what everybody wore there.

Edit: It's difficult to provide sources here, just because your question is so general. There are plenty of books detailing fashion or society of both eras. If you're interested in specific recommendations, you'd have to ask a more specific question.