In addressing why American Christianity seems to gravitate toward the political right, there are at least two big issues worth talking about. One is the attachment of white evangelicals to the Republican Party. The other issue is the demographic decline of Christian groups that were politically moderate or left-wing. Both of these are more recent than you might expect.

White Evangelicals Move to the Right

I’ve written about on bit on AskHistorians before about the connections between white evangelicals and the Republican Party, and particularly white evangelicals’ choice to abandon their fellow white evangelical Jimmy Carter in the election of 1980 to support Ronald Reagan.The short version of this history is that there were evangelicals since the presidency of Franklin Roosevelt who had distained the Democrats, social programs, and political liberalism. Yet there was quite a bit of political diversity among white evangelicals too, which is how Carter could be a viable political candidate as late as 1976.

By the late 1960s and 1970s, white evangelicals in the south were particularly upset about the civil rights movement and the antiwar movement’s efforts to end the conflict in Vietnam. Starting in the late 1970s, ending abortion and stopping gay rights also emerged as major foci for white evangelicals. Their stance on all these issues aligned white evangelicals with the Republican Party.

By the 1980s, a number of evangelical leaders, like Jerry Falwell, and Republican political operatives, such as Paul Weyrich, created a series of organizations intended to mobilize white evangelicals to vote Republican. The Republican Party also began to cater to white evangelicals more and more; in the Reagan presidency, they were given considerable access to the president. Since the 1980s, white evangelicals have been a reliable voting block for the Republicans.

The Decline of the Mainline

The move of white evangelicals to the right happened at the same moment that mainline Protestantism began to considerably decline. Mainline Protestants, a group including Episcopalians, United Methodists, Presbyterian Church USA, the Congregationalists, Disciples of Christ, Evangelical Lutherans, and American Baptists, had considerable cultural power in the U.S., even making up a kind of unofficial religious establishment. They were largely white, politically mixed, and often institutionally leaned leftward. In 1970, the Protestant mainline made up about 30 percent of the U.S. population; today they have declined to a little under 15 percent of the total population.

Why mainline Protestantism declined is hard to pin down precisely, but there is a case to be made that part of the reason was that it became too politically left for many Americans. The mainline was active in support of civil rights and ultimately critical of Vietnam. These churches also embraced women in ministry and many of them began to support gay rights in the 1980s and 1990s.

The Protestant mainline still exists and continues to include both Republicans and Democrats. But the fact that it makes up far smaller a percentage of the population means it has considerably less cultural influence. It also used to have a substantial role in politics. Lyndon Johnson froze the National Council of Churches out of access to the executive branch because he viewed them as being too critical of the war in Vietnam.

Isolationism

The one thing I’d quibble with in your question is that I wouldn’t describe white evangelicals as having been “isolationist.” While they were a part of the political right, they tended to favor foreign intervention and expanding American military power. They saw the Soviet Union as a threat to Christianity and believed military force was essential to defending the U.S. and Christianity from it.

In Vietnam, white evangelicals were among the war’s biggest champions. During the Reagan Administration, they tended to support U.S. intervention in Latin America. Evangelicals were critical of Bill Clinton’s efforts to use military force for humanitarian ends, but this didn’t involve support of isolationism. There were a few fundamentalist predecessors of white evangelicalism in the 1920s and 1930s that might properly be called isolationists, but the movement as a whole never really embraced this direction.

It’s worth observing that isolationism really wasn’t common on the political right through much of the twentieth century. I’d argue Robert Taft was probably one of the latest champions of this stance, and he died in 1953. White evangelicals largely mirror the mainstream Republican views on foreign policy.

Lots of Christians on the Left

I’ll end by observing that both political parties still are primarily made up of Christians. Black Protestants, with religious views largely indistinguishable from white evangelicals, are a core base of the Democratic Party. In the south, since the Civil Rights Act, they have been critical to the Democratic Party's continuing viability. Roman Catholics are roughly evenly politically split between the major parties.

Since the 1970s, there are more white evangelical Protestants who are Republican and fewer mainline Protestants supporting liberal causes, but both sides still exist. White evangelicals tend to be fairly visible in popular media and news, so people are often unaware of the considerable political diversity among American Christians.