The answer seems obvious. Communities all over the country are becoming less isolated from each other. Television, the movies and pop culture are homogenizing American attitudes. Population movement has exposed more people to different views. Religious educational institutions are now open to all people.
Religious intermarriages are on the rise.
Also people are less dogmatic about the finer points of their faith and, therefore, various denominations are building closer ties with each other. More people than ever have been exposed to scientific thought which causes them to take a broader view of religion. That is, orthodoxy is giving way to more liberal practices. It is not that the flock changes its mind about its belief. It is that as older members die, they are replaced by younger more liberal members.