The United States has a long and proud revolutionary tradition starting with the revolt against the British, continuing with the abolitionist movement, the workers' struggles in the 1930s and the civil rights movement. Much of this, of course, is not taught in American schools, where instead of looking at the role played (for example) by trade unions, the Communist Party and radical agitators in pressuring reforms, as well as the ruling class' fear of the Soviet alternative, children are taught to view policies such as the New Deal as something that presidents (in this case, Roosevelt) gave to the people out of the sheer goodness of their hearts.
I'd say there are two main reasons for the negative perception of socialism in the United States today. One is simply the cumulative effect of a century of anti-communist propaganda that utterly demonizes any alternative to capitalism. Even though societies like the USSR and the People's Republic of China achieved massive economic growth, industrialized backward countries, virtually eliminated illiteracy and drastically reduced inequality while improving the status of women and offering full employment and a range of social services such as free health care and education, we are trained from childhood to focus only on the negative aspects of those societies.
It's important to note that anti-communist orthodoxy generally treats the more repressive elements of those societies as something inherent in communism itself as an ideology. The elephant in the room that nobody ever talks about is the role of imperialism, particularly U.S. imperialism, which wages non-stop campaigns of destabilization against any country perceived to threaten its interests (particularly one touting an alternative economic model).
For example, immediately after the USSR was formed in 1917 -- at which time it was already in a poor state due to the effects of the First World War -- it was immediately invaded by more than 20 foreign armies in addition to the opposing Russian White armies. When you've fought a world war and a civil war that devastated your country and left millions impoverished and hungry, these are hardly the ideal conditions to start constructing socialism, which by definition implies the existence of plenty rather than scarcity. If you're trying to distribute items in a society dominated by scarcity, the role of the state will inevitably increase.
From the Trotskyist point of view, as well, it's impossible to construct "socialism in one country" (as Stalin argued) because every country is connected to the world economy. The Russian Bolsheviks after the October Revolution were depending on assistance from an advanced capitalist country that would have its own revolution -- in this case, Germany. When the German Revolution failed in 1918, the USSR was left to its own devices.
Anti-communist propaganda, of course, deliberately ignores all this context. Instead, communism is portrayed as inherently gray, dreary and oppressive, with leaders who apparently sit around plotting how to starve as many of their citizens as possible. I should just add that if one were to treat "capitalism" as an ideology in the same way that we treat "communism" you would be laughed out of the room. When we talk about famines in China, people will say "Mao killed 30 million people!" (the professional anti-communist can make up any ridiculous number they want with the assurance they they will rarely be asked to justify them). The equivalent would be saying that the tens of thousands of people who die in the United States every year because they can't afford health care were personally murdered by the president.
(If we were to tally the deaths that one could attribute to "capitalism", of course, the numbers would absolutely dwarf numbers caused by "communism". How many people died due to slavery, colonialism, or in the two world wars that were caused by the desire of rival powers to divide the globe in their efforts to control markets and resources? How many children even now die of starvation or from easily preventable diseases every day? You get my drift.)
The other major factor in negative connotations of socialism in the United States -- and intertwined with years of anti-communist propaganda -- is the fact that the United States for much of the 20th century was the world's richest nation, particularly during the boom that resulted from the opening of new markets after its victory in World War II. Like other Western nations (which were richer to start with because of their imperialist exploitation of the rest of the world), workers in the United States had agitated for better conditions and wages in the previous few decades, and after the war the ruling classes in those countries were even more likely to accommodate those demands for fear of the Soviet counterexample. So it was in that period that you saw many progressive developments such as public pensions (Social Security), public health care (Medicare and Medicaid in the U.S.) and poverty reduction programs.
In such conditions, with the average family able to afford a car, a house and a vacation once a year, it was easy to think that this was the new normal, that things would always be this way. At the same time, any threat to the existing order could always be countermanded by pointing to the more unsavoury aspects of Stalinist governments and saying, "You want socialism? There's socialism for you!" Better the devil you know, the thinking went, to the point where even much of the "left" ended up buying into the anti-communist orthodoxy. Today, of course, with capitalism in crisis all those hard-won gains from the past are being clawed back in the name of "austerity".
There's one other factor I forgot to mention. Whenever radical left movements have cropped up throughout the history of the United States, more often than not they have been greeted with savage violence in partnership with the state. In the South under segregation, for example, the Communist Party attracted huge numbers of black supporters, which led to the state partnering with the Ku Klux Klan to brutally repress these movements.
Similarly, the rise of the Black Panther Party in the 1960s was seen as a radical threat by the state and its repressive arms such as the FBI. While often mischaracterized as "black nationalists", the Panthers were in fact revolutionary internationalists heavily influenced by Mao. One of the ways that the state dealt with the threat of the Panthers was through harassment, intimidation and the physical extermination of its leadership (look up the tragic fate of Fred Hampton), which helped weaken such radical organizations over time.
Even today, the state is remarkably quick to target and destroy movements it perceives as a threat. Just look at how quickly the Obama administration co-ordinated law enforcement agencies across the country to destroy the Occupy movement.
So I guess that's how I would summarize the three most important factors in the negative perception of socialism in the United States -- the ideological, the economic and the coercive. That said, the onset of the economic crisis in 2008 has dramatically shaken many people's previous assumptions and you never know what the future may hold...