"Construction of religious buildings in the U.S. has fallen to the lowest level at any time since private records began in 1967. Religious groups will build an estimated 10.3 million square feet this year, down 6% from 2013 and 80% since construction peaked in 2002 ...Possible trends include more multisite churches, including using rented facilities for branch locations.
"Behind the decline is a confluence of trends: a drop in formal religious participation, changing donation habits, a shift away from the construction of massive megachurches and, more broadly, a growing taste for alternatives to the traditional house of worship."
The biggest factor is the drop in participation. In 1972 9.3% of Americans said they never attended religious services, compared to 25.3% in 2012. Over the same period those describing their religious preference as "none" went from 5.1% to 20%.