Guesses on why, yeah. Don't actually know though.
But I think the reasons vary with locality, too. Not just one answer for everywhere. For instance, there are big areas just out of town here, that were that thick with rabbits in the mid to late 70's. And old timers tell me they had been thick around those parts forever. Then they crashed in the 80's, and they have never come back nearly that strong in those areas since. A lot of that area, it's been kind of unusual to see a jackrabbit for almost 40 years now.
While in other areas, like where that video clip was taken, I've seen them come and go in varying waves of population density quite a few times in that same time period.
So, for one thing, I don't buy the seven year cycle thing, at all. I've never seen that hold up, anywhere. Yes I have many times seen populations get really high and then crash, then come back again. But not ever on any regular timeline. Sometimes they are back in only three or four years if conditions are just right, other times they don't come back for fifteen or twenty years before the right weather and habitat come together for a few years in a row.
The usual suspects, for crashes and rebounds - weather and disease. Jacks seem to be pretty susceptible to the weather. Heavy snow, especially. I've seen healthy populations disappear with one winter of heavy DEEP snow. I think their food is hard to get to, their breeding isn't very successful and they are easy pickings for predators. They also yard up really tight in heavy snow so any disease gets pass around quick.
Drought, I've seen them thrive the first few years of a drought. But a many years long one, like we are in now, the habitat just degrades so much, nothing can thrive in it. The exact spot where that video was shot, is a barren wasteland now. I don't see rabbits ever coming back strong in that area.
And of course, disease is a big deal with jackrabbit populations.
But, so, why they seemingly NEVER come back in a lot of areas, my guess, it's a variety of mixtures of all of the above. Combined with habitat loss, from whatever cause - development, farming, drought, fire, etc. Once they are really down, they need several consecutive years of just the right conditions to really bounce back. And it seems in many areas, there just hasn't been several years in a row of those conditions and maybe there never will be again.
- DAA