Yes, there are many exceptions of rural or small-town Unitarian, and Universalist, churches.
But in 1940, there weren't a lot of suburban Unitarian or Universalist churches, because there weren't a lot of suburbs.
Suburbanization and Housing Segregation are the same process, and could be the most important transformation of the society in the last century. We now know that it was a process driven by powerful financial and governmental policies. The white urban working class was offered the chance to buy new homes in the suburbs with subsidized and guaranteed credit, which allowed them to begin accumulating family wealth. The black working class in the city was denied this opportunity, and whole sections of cities were denied credit for new or improved housing. Vast profits were made in housing industry, the financial industry, much of which was directed back to the suburbs as local government revenue.
Some urban Unitarian churches followed their people to the suburbs. Some did not. My opinion is that it did not matter; a church located in the city, but with a suburban congregation is a suburban church with less parking.
Suburbanization affected the theology and the ecclesiology of liberal religion. For the most part, current Unitarian Universalism is liberal religion customized for suburbanites.
Suburbanization atomizes community and demolishes culture.
The old urban church was embedded in the urban community. Its minister was a recognized leader in the city's life. Congregants not only belonged to the church, but also to many other civic organizations. They belonged to the Y and participated in local politics. Political leaders knew how many votes could be moved by an appearance at a particular church. There was even a time when a provocative sermon would be reported in the local newspaper.
[A member of the church in Worcester, who was deeply connected the civic life of the community thought it odd that he should invite a friend to come to church. He knew lots of people and he assumed that they either had a church, or chose not to go to church. Church was part of people's private, personal life, quite apart from the network of public relationships in which he lived.]
Life in the suburbs changed the role that the church played in people's lives. The church, over time, became, for many people, their primary community. In many cases, people know more people in the congregation than in their neighborhood, or through any other non-work connection. And even those connections through the congregation are weaker, because congregational friends live in other suburbs and their kids go to other schools.
Theologically, in liberal religion, the ultimate values becomes "relationship" and "community." Ecclesiologically, in liberal religion the most important purpose of the congregation becomes to create and sustain "community." In terms of public theology, the suburban church blends into the de-politicized landscape of the suburbs. Public witness becomes going into the city to participate in witness actions there.
Whereas in the past, the church was united by its general message to the community in which it was embedded, now the goal of the church was to be the community.
[Some of the elders in the church I served in Worcester were surprised that other people wanted to know everyone in the congregation. They thought that just because some other people agreed with the message of Unitarianism didn't mean that they were meant to be friends. These elders had their friends and they assumed that the others had theirs, as well.]
The suburbanized church tends to elevate the values of suburbia: focus on appearance, cultural similarity, the aspiration of a completely safe environment, convenience, good customer service, self-containment. The model is a shopping mall.
And finally, whiteness. Suburbanization, at the ground level, is white people moving away from black people. That is the practical effect of government and financial policies and it occurs without conscious racist thought. You want to buy a home in a more prosperous suburb, chances are it will be a whiter suburb. As white people move away from black people in the suburbanization process, the very presence of black people signals potential danger and downward mobility. The predominantly white suburban church will inevitably view black visitors and potential members with suspicion.