While the big knives were generally more popular pre-1870, that does not mean they were not used later:
1) Cap and ball revolvers contiued to be used well into the 1880's and even later by some
2) Major knife makers such as J Rodgers and Russell, all continued to catalog 7-10" length Bowies through the entire western frontier period (up through the 1890's) so there was a definite market for them. See some of MJB's earlier posts on suppliers in whihc he inluded blade lengths.
Some how the idea that a knife should be a multi-purpose tool is IMO part of the "modern" mindset amongst many re-enactors, when in fact during the period there were fighters including Bowies, butchers, skinners, camp knives, pocket knives (often carried even when carrying a big knife). While many of these types can be used mult-purposely they were often purpose designed. Like anything else using a big knife is a learned skill. On the other hand many (if not most, especially the commercial models) of the modern repros of big knives are too thick without the proper tapers, etc. and too heavy making them awkward to use. Yes there were period built big and awkward knives (i.e. many of the D guards from the ACW period), but in general the period big bowies/side knives made by the English for instance are well balanced and not overly heavy or awkward to use. Good for skinning game, etc? Not necessarily. but then they weren't designed for such uses anymore than a 16" butcher knife was designed to peel carrots.......
Another part of the "story" is as always who, when, and where you are from - in the SW and California for instance where there is a heavy Spanish influence, big knives (many belduques are over 10" with thick spines) continued to be widely used throught the period more than other parts of the country.