The reason why has to do with the rise of “Movement Conservatism” after the Goldwater campaign of 1964. Once upon a time there were commonsensical Republicans, known as “Rockefeller Republicans” (not for their wealth but because Nelson Rockefeller was a prominent one), socially liberal, pro-civil-rights-movement, moderately conservative on fiscal and economic matters. But the Movement Conservatives – forming a coalition of various very dissimilar factions (Commie-fighting warhawks, small-l libertarians, social-religious conservatives, nativist-populists, Midwestern conservatives who wanted the whole New Deal rolled back to zero, white Southern conservatives reacting to the civil-rights movement, and always business-interests conservatives above all) who more or less agreed to work together for common goals in a “No Enemies To The Right” strategy – took over the party, mounted the Reagan Revolution in 1980, and gradually marginalized the Rockefeller Republicans. You can read the story in The Right Nation: Conservative Power in America, by Adrian Wooldridge and John Mickelthwait.
And now, just these past three years, the Tea Party emerged, which still finds the GOP insufficiently radical-conservative and is trying to push it further right and succeeding.