Conservative evangelical and pentecostal churches did not decline. The African American Protestant church did not decline. I don't know about the non-English speaking Catholics.
Most of this decline occurred during a period of ferocious cultural conflict in the United States. There was a powerful backlash against the ideological breakthroughs of mid-century liberalism: anti-racism, feminism, the LGBTQ movement.
One aspect of the conservative backlash was a theological and ecclesiological attack on the mainline Protestant churches and denominations that had embraced (tentatively) those ideological breakthroughs.
There were several strands of the conservative attack on liberal theology, but they came down to this: liberal theology was too open to influence from "the culture" and could not defend Christian morality against the forces of cultural "decay."
In terms of the forms of the church, the conservative Christian movement was developing a new and more culturally congruent way of doing church. They stepped out of denominationalism and created the non-denominational 'bible-centered' church. And because their more entrepreneurial ministers were building many new churches, they had a greater freedom to experiment with more popular and contemporary liturgies, music and preaching.
Far-right foundations poured money into institutions like the "Institute for Religion and Democracy" which organized and funded conservative opposition groups in three large mainline denominations: the United Methodists, the Presbyterians and the Episcopalians.
The liberal-leaning mainlines froze like deer in the headlights. They vacillated between imitations and inaction. Most of the mainline Protestant denominations entered into long struggles over the ordination of gays and lesbians and marriage equality. Individual churches got mired in disputes over contemporary vs traditional music and worship. Meanwhile, members headed for the exits: many moving toward the more dynamic conservative churches and others dropping out of organized religion altogether. But as many that left, those who were left were still divided.
Meanwhile, the now dominant conservative church ran the brand of Christianity and all of organized religion into the ground. Among the young, polls tell us, respondents associate "Christianity" and
"Religion" with rightwing politics, homophobia, moralism and hypocrisy.
It is no wonder that now the evangelicals are talking about the growing cultural irrelevance of the church, as though this is some mysterious thing. Two wars, escalating inequality, uncontrolled racist policing and mass incarceration have turned the country as a whole against the conservative project, and that includes the conservative church movement.
The tide is turning. People are angry about the economic system and have realized the terrible error of enriching the wealthy as a plan for economic growth. Social movements have been growing; progressive political forces are feeling strong and confident. 2015 will see continuing popular mobilizations about a wide variety of issues. 2016 will be dominated by the election. (Odd = Protest, Even = Vote).
The main thing for liberal religious institutions is to get connected to the rising tide of change.
You can be pews, organs and robes. Or you can be jeans, rock and sit on the floor. You can be Christian, or humanist, both or neither. How you do church will shape who you do church with, so choose carefully. But however you choose, you must be an accessible and holistic path to participation in the rising movement for social, cultural and political transformation in the country. And you must be able to explain how and why you are.