It started as a tactic and turned into an industry

By the 1990s the Republicans had a formula for beating old-school postwar Democrats (call them soft on defense, soft on crime, pandering to "special interests" (unions, minorities, feminists, etc.), "anti-business" ). That formula wouldn't work against Bill Clinton, "pro-business" governor from a "right to work" state who didn't follow the "big government" narrative and was willing to go against "special interests" in his own party.

The Bush campaign fought this in two ways: 1) dig up whatever dirt hey could in Arkansas and 2) forget reality and just portray him as the conservative voter's ultimate bogeyman (draft-dodgin' dope-smokin' citizenship-renouncin' Baby Boomer Hippy Bill and his feminazi "partner", Homemaker Hater Hillary). Then, in a classic case of Republican Election Delusion, they were stunned that Bush lost.

Enter Rush Limbaugh: he devoted his radio show to rallying conservatives, saying Clinton was only "technically" president because he won by a plurality, painting him as holding the office of the president illegitimately. So, for the red-meat crowd, anything to remove the pretender from office was legitimate. For the more strategically-minded conservatives, a constant stream of investigations and scandals would be a drag on any Clinton attempt to revisit old Reagan-Bush scandals now that the investigatees no longer in in a position to stall investigators. And it would hamper attempts to reverse some key Reagan-Bush policies.

Add to the mix a media eager to show that it didn't have a "liberal bias" (one news magazine had a "Clinton suck-up watch" to chide any coverage that was seen as too favorable), and who grew up on how Watergate coverage created superstar journalists -- and were eager to get their turn.

So you had a conservative public stoked to believe absolutely anything about the Clintons (and eager to get the juicy details about their perfidy), a growing conservative media counterculture that was willing to supply them with "what's REALLY going on," a political establishment that saw advantage (and, eventually, possible revenge for Watergate) in nonstop attacks and a media that saw a potential career jackpot in becoming the new Woodward and Bernstein. And after the 1994 election ushered conservative true-believers into a House majority under the bomb-throwing banners of Newt and Rush, the brakes weren't just worn out, they were sawed off.

They've been operating under that system ever since.