It's because the Right is, as Glenn Greenwald so aptly put it, "a pack of crazed, primitive radicals who don't really believe in the country's core founding values".
I can't deny that in the South in particular there are whites who can not stomach a black guy named Barack Hussein Obama being president (after all, most elderly Southerners graduated from segregated high schools--Jim Crow was still not so long ago). All the same, I think pretty much the same level of roiling vitriol would most likely be occurring if Hillary or some other Democrat were in the Oval Office. The Right automatically dials it up to eleven when a Democrat dares usurp "their" presidency; there's no room, really, to get any more exercised on account of race than they already would be on account of party affiliation.
After extensively referencing the unhinged actions of the right during the Clinton presidency, Greenwald nails it in his conclusion:
Nothing that the GOP is doing to Obama should be the slightest bit surprising because this is the true face of the American Right -- and that's been true for a very long time now. It didn't just become true in the last few months or in the last two years. Recent months is just the time period when the media began noticing and acknowledging what they are: a pack of crazed, primitive radicals who don't really believe in the country's core founding values and don't merely disagree with, but contest the legitimacy of, any elected political officials who aren't part of their movement. Before the last year or so, the media pretended that this was a serious, adult, substantive political movement, but it wasn't any truer then than it is now. All one has to do is review their behavior during the Clinton presidency -- to say nothing of the Bush years -- to see that none of this is remotely new. Nothing they're doing to Obama is a break from their past behavior; it's just a natural and totally predictable continuation of it.
Matt Yglesias also makes an important point:
I think the crux of the matter is that since 1928 or so, the Democratic Party has typically presented itself in national politics as representing a coalition of "outsider" groups—Catholics & Jews back in the day, nonwhites and seculars more recently. The actual identity of the leader of the coalition matters, but only at the margin. It could be a patrician from upstate New York or a war hero from South Dakota or a cracker from Arkansas at the top of the ticket, but fundamentally no matter who’s in charge the election of a Democrat represents the mainstream’s loss of power to the outsiders.
Hopefully this will be a wakeup call to those who bought into the idea that strife between left and right would melt away due to Obama's power as a unifier. Obama (with Axelrod's guidance) cannily sold himself as just such a unifying figure (not sure if he/they really believed it, deep down); and tons of people--young people especially--drank the Kool-Aid. I feel like many of us who have been around politics a little longer knew better (although, to be clear, I voted for Obama in the primary--not with stars in my eyes, so much, but because I decided he was our strongest standardbearer).
Remember when Hillary scoffed at "change" in the sense the Obamaphiles meant, pointing out that any Democrat would bring much-needed change from Bush? Obama insisted that to the contrary, a Clinton restoration would bring back the insanity of the Vince Foster/Whitewater/Monicagate imbroglios, not to mention the left-right struggles that baby boomers have been waging since the 1960s, while he would take a higher road, unencumbered by that baggage. I think it's clear now that Hillary is vindicated in that argument: no grand, dreamy unification is possible (nor is it desirable, in my view), and we need to just accept--embrace!--our partisan status as progressive Democrats and duke it out in never ending street fights with right wing Republicans who will never relent no matter how we reach out to them. "Reaching out", in fact, should be done only as a political tactic to appeal to swing voters and look reasonable (which appears, fortunately, to be just what Obama is doing now).
As I say, I do hope that all this is a wakeup call, a splash of cold water in the face, for those who (naively in my view) believed Obama to be a transcendent healer of the body politic who would bring us all together in harmonious coexistence. But I fervently hope that those same idealists, who now discover those lofty notions were a pipe dream, do not now withdraw from politics and become apathetic. Instead, they need to understand that the rabidity of the Right means that the rest of us have to stay all the more focused, vigilant, and strong, and give no quarter to the forces of darkness baying at the gates.