They are reintroducing themselves. They disappearred because there was a family on every 40 acres that would shoot them (and the deer they ate) on sight.
Whitetail deer were also hunted to extinction in most of the U.S.
What’s different now?
1. Very few rural Americans.
2. A thick and well fed deer population
3. Currently no wolves to compete with
I think the real pressure that thinned them was hunting them with dogs. If you were to go out and either stand or attempt to stalk mountain lions, you’re probably not going to have much luck, ‘least for older ones. If they were around and you were using dogs, once they got the scent you were probably going to get one.
I haven’t seen the one at my property in Nevada (haven’t spent a lot of time there yet though), but I have seen lots of its sign, and one of its old kills.
Now that everyone has game cams, cell phone video and even night optics, that, along with no pressure and lots of deer will probably see sightings demonstrative of a rapid repopulation by them of eastern states.