Compare the parties now to 30 years ago and that's why.
Explain what you mean; this is an incredibly lazy answer. I'll give you the benefit of the doubt that you're not a college-aged "parties switched" moron who hasn't done his own research. 
I appreciate the benefit of the doubt Rockefeller. If you look as far back as 1952 you can see Stevenson, then Kennedy, and even Humphrey as much further to the right than their contemporary counterparts of Hillary Clinton, Bernie Sanders, Dennis Kucinich, etc. Just before then you had Thomas Dewey a liberal Republican who had accepted parts of the New Deal. Nixon, Eisenhower, Ford, Rockefeller, Scranton, Romney, and to an extent Bush Sr. represented the Northeastern liberal Republicans. There was Nixon's quote of "we're all Keynesians now." However, something started in 1964 that took half a generation to come to fruition and the groundwork began with Barry Goldwater. He was strong on states' rights which appealed to the deep south. It grew from there into an idea of limited federal government not only on social matters, but also economic matters which he and Reagan were very supportive of. Reagan has long been held as the conservative prophet who took it above and beyond. At the beginning of his presidency you still had your moderates such as John Anderson, Bob Dole, Bush Sr., but by the end of his presidency people like Pat Robertson were taking the stage. Social and economic issues have driven them to the right.
Now as for the Democrats, there Kennedy's famous quote of "ask not what your country can do for you, but what you can do for your country." He also lowered tax brackets. In those days you had your segregationists and your states' rights Democrats such as Stevenson and Wallace as well. The Civil Rights Acts and expansion of redistribution of the wealth really took off under the Johnson administration though. The hippy movement cast a negative light on such ideology, but by the 90's when the hippies had grown up, it wasn't as laughable as it had been half a generation before. Now today's Democrats are a bunch of old hippies, atheists, free loaders, drug addicts, uninformed college students, single mothers, and minorities. They've fallen far from the days of Kennedy to Hillary Clinton actually telling voters to ask themselves "what's in it for me?" Obama is the new face of the left which is openly trying to "fundamentally transform" the US to quote the mentioned person. I have to ask the elected president what's so bad about our great nation that it has to fundamentally transform?
In order to fundamentally transform something, you must transform it's foundation first. For the US it's our inalienable rights endowed by our Creator. Creator is capitalized because it refers to a deity. Let me remind our college age hacks on here that freedom of religion doesn't mean freedom from religion. Your right to be an atheist isn't part of our rights in that regard. In 2012, the word "God" I don't believe was mentioned once throughout the DNC. This will only get worse. Their first attempt at fundamentally transform the US is getting rid of religion and any idea of God. Once that's done we are fundamentally transformed. Eventually there will be a generation of people who aren't taught in public schools that we were once a nation of religion and freedoms. For now the left's higher power is Obama but it will only become more intrusive as more Democrats greedy for hunger and power infest our freedoms with notions of Obamacare and global warming. This is only the beginning. They've got the power and we have the numbers. Sometimes I wonder how much better we'd be if the Democrats were on board for fighting terror instead of God and American values.
