Why was Idaho so democrat?
Once upon a time, LDS members did not vote as a block for one party. And yes, mining/lumberjacking faded in importance, and yes, then lots of new folks moved in, fleeing places in which they no longer felt comfortable with, e.g., the changing demographics, in particular CA, and well, they didn't vote Dem typically. And then the farm workers in the Snake Valley potato growing belt turned Hispanic, and often temporary guest workers or illegal, and so that demographic cohort just ceased to vote, unlike perhaps the ones they replaced that used to do that kind of work, and when they did, tended to vote Dem. However, as most counties in Idaho moved Pub, with the LDS zone in the SE moving very, very, Pub, one county moved the other way, and in a quite dramatic fashion, which also happens to be by far the richest county in the state. No doubt you can guess which one, can't you, and why? Of course you can! (In fact, counties of its ilk trended Dem right across the Fruited Plain, almost without exception.) 
Democrats hold the Governorship continuously from 1971 to 1995 (Andrus, Evans, Andrus again). Frank Church would have defeat Symms for reelection in 1980 if not for the early projection for Reagan, which depressed the Democratic turnout in Idaho (a prime example of results from early states influencing voting in those, where polls opens later).
So yes, Idaho was quite friendly for Democrats on state and congressional level before 1990s. It was diffrent with the presidential, as LBJ is the last Democrat to carry the state (albeit narrowly) back in 1964, which was the first time since Truman sweep of the West in 1948.
Beside of factors mentioned by Torie, Governor Phil Batt, elected in 1994, is frequently credited with rebuilding the Republican machine and helping to turn the state into virtual GOP dominance.
Walt Minnick was a fluke. 2006 gubernatorial election was
relatively close (for Idaho standards), but that was 2006.
Republicans are more than safe there.
