Theology may not be the most important factor. Population changes are part of the answer. Churches in dominant and denominational positions are vulnerable to the supply of persons suited to the label. If no Presbyterians are moving into our town, then the Presbyterian church will likely be declining. The movement of people is one ingredient.
Societal changes are also part of the answer.
I am inclined to think that the biggest factor is something much simpler. Dominant and denominational positions are inherently passive. Distinctive position is active. But society changed, and with it changed the role of religion in American life and the position of churches in society. Now the church is marginalized from the mainstream of society, and social forces move people to other institutions, movements, causes, and activities.