Michele Bachmann's presidential campaign imploded. Rick Perry's did the same. Herman Cain ran into trouble. Now, Newt Gingrich has become Republicans' favorite conservative alternative to the moderate Mitt Romney, and political analysts aren't wondering if Ginrgich's lead will crumble, but when. Why can't any of the non-Romneys stay on top? Here, four theories:

1. The endless debates expose everyone's flaws
The Republican presidential candidates are slugging it out in an unprecedented series of debates — "10 so far, with another 12 scheduled," says Ewen MacAskill at Britain's Guardian. All that exposure has "changed the dynamic of the nomination race, making and breaking candidates." Rick Perry's poor debate performances sunk him. On TV, every weakness is amplified. Millions of viewers tune in to every installment of this new brand of reality TV, where they can curl up on the couch, "enjoyably awaiting the next gaffe."

2. Intense media scrutiny reveals their shortcomings
Republicans like Perry and Cain have more than conservatism in common, says Adam Silbert at Politico. They're also "newcomers to the national political stage." And once a candidate emerges from the cellar and starts rising in the polls, says Jennifer Rubin at The Washington Post, suddenly he starts receiving more scrutiny. That means "sharper questions" and "more barbs" from opponents. Most of these guys just couldn't handle the pressure. Now, "with time running out before the Jan. 3 Iowa caucuses," Silbert says, "a frustrated Republican Party seems willing to revisit its customary formula of nominating" a known candidate who has "paid his dues and waited his turn" — the "smart, familiar Gingrich."

3. Clearly, the media is trying to help Obama
"The mainstream media keep pushing alternatives" to Romney because "they are terrified of running against him," says Ann Coulter at Townhall. Liberal journalists just want to keep Republicans fighting. It's obvious now that they're trying to sell us Gingrich, the former House speaker who had an affair while trying to impeach Bill Clinton and called "Paul Ryan's plan to save Social Security 'right-wing social engineering.'" Conservatives need to stop being purists and get behind Romney, because he's "the only Republican who has a shot at beating Obama."

4. The GOP field is just abysmal
There's no mystery about why every conservative flavor-of-the-month has gone down in flames, says Markos Moulitsas at The Hill. Bachmann's baggage was her "craziness"; Perry's was his "utter failure to string two sentences together"; Cain's fatal flaw was his "comically bad" handling of sexual harassment allegations. The only reason Republicans are embracing Gingrich, whose pile of negatives made them laugh him off before, is that "it's late in the game and they're desperate." Really, "there's no greater indictment of the GOP field than the fact that while voters might be desperate for a Romney alternative, no permanent host can be found.