According to recent research conducted by Pew Research Center there are several reasons for the proportional decline of mainline Protestant religions. The first is a generational shift, with mainline Protestant denominations appealing mainly to an older demographic. Millennials tend not to join mainline Protestant denominations.
The next reason for the decline is low retention rates. Over half the people raised in these churches tend to leave them as they become adults. In the past decade, a larger number of people have left these denominations than have joined them.
Another reason cited by Pew is the increasing ethnic diversity of the United States. While mainline Protestant denominations draw members mainly from the white/Caucasian ethnic group, this group is declining as a proportion of the overall population.
This decline is uniform across mainline denominations, with the singular exception of the Baptists who have grown from 1.9 to 2.1 percent of the population in the interval from 2007 to 2014.