This is Montana specific, but it might be part of the reason there is more pressure on ranchette purchases, and therefore migration to other states, such as Idaho, the state mentioned by the OP.
If there is one thing that makes Montana a bit less attractive than other mountain states, it goes back to the 1994 Legislative session when the Legislature passed a law that precluded any subdivisions less than 160 acres unless it went through full country subdivision review. A deadline was given and some where grandfathered by having applied before that deadline. Most of the ranchette subdivisions we see in MT were grandfathered and a small number were actually approved.
As such, the 5, 10, 20 acre ranchettes I see in greater frequency in some states I travel to are less common in Montana. It is a lot more expensive for someone to come to Montana and buy 160 acres when they can go to places in CO, AZ, NM, ID, NV and WY and buy some of the much smaller acreages and still feel like they have their "piece of paradise." That forces folks moving to Montana to have less ranchette options than in other states and make a choice; 1) buy in town (which does drive up costs in town) 2) bite off 160+ acres and go into debt to do that even with the big cash out from selling the big city house or 3) look at smaller acreage options in other states.
Many times I am driving and see the ones that did get grandfathered in MT or I see the landscape messes in some other places I travel, and it makes me thankful for the enlightened folks of that time. Remarkably, a big part of that effort came from ranchers and farmers who stood to make money by subdividing into postage stamp patterns. By promoting that legislation and giving it the critical support it needed, these landowners left money on the table and all of us are better off for it. Part of why I always remark at the land ethic the majority of Montana landowners have.
Add to that, Montana has higher income tax rates than Wyoming, Nevada, and Idaho. Yeah, we have no sales tax, but unless you spend all of your income, a sales tax is more attractive to retirees than an income tax. If you are moving somewhere to retire, those tax issues weigh into the decision, though any of these mountain states seem like a "tax haven" to those migrating from California. Not advocating for more taxes as a means to discourage migrants from coming, but there are some aspects of Montana that make the migration less than it otherwise would be.