Very simple answer.
pas·sé
adjective
1. no longer fashionable, in wide use, etc.; out-of-date; outmoded
Why did this happen? When I grew up, myself and all my friends wanted guns like the TV cowboys had, six-shooters, especially the ones that took "caps" so they went BANG! Anyone raised like that, and who went through the stage of pretending to go around shooting the bad guys, might really be thrilled when they could get the real thing.
Fast forward to today. If you watch on TV, or the movies, or most of the magazines, the guns that most of the heroes have are something that looks much more sinister, and that's what people nowadays are likely to want.
If guns can have "charm", I think that feeling fits revolvers best. Maybe it's just my upbringing. Semi-autos make me think of the military, current entertainment stars, and maybe even with attached accessories such as lasers, flashlights, night vision - all the new stuff, that gets kids and younger people excited - much more so than the old fashioned stuff that was used to protect the good guys from the rustlers and Indians.
Of course, you can apply this to the likely future, when both revolvers and the current semi-automatics will become passe, replaced by computerized guns with electronic ammo that will hit the bullseye every time, won't work for anyone who steals it, and will probably be constantly "on-line". Laugh now, but 50 years from now, that's what I expect will be quite common.....