Once when I was a missionary district leader, one call to my zone leader went particularly badly. I was trying to get permission for my district to take a hike in the woods, essentially. (The difference between a hike in the woods, and essentially a hike in the woods, was the sticking point.) During the conversation, the ZL and I figured out that my district was still taking their weekly day off from missionary work on Mondays, when the rest of the mission had moved to Wednesdays a month earlier. My district, on the far southern edge of the mission, at a time when I was trying to keep interactions with my ZL to a minimum, hadn’t gotten the memo. For a month, the “isogloss” of missionary practice of holding P-day on Wednesdays didn’t include us; we were a relic area preserving an older form.
In linguistics as in religion, isogloss boundaries change over time. If two people can communicate with each other, eventually they can end up sharing not only information, but also linguistic habits. The same seems to me true of religious habits within the Church as well; we adopt practices and beliefs not only through indoctrination but also by imitating each other. Correlation is, in a sense, no different than any other innovation in religious practice and belief. From the center of innovation within church administration, correlated practices and teachings propagate effortlessly and completely along the top levels of church leadership, but their progress slows somewhat after that. Eventually most members of the church get the memo, but a few never do, producing relic areas of former practice and belief. Sometimes language barriers hinder transferal of instructions, some people are no longer listening to anything their bishop says, and some places are just remote. (The same feature can be preserved in different places for vastly different reasons, however, so it should not be assumed that relic areas conserving the same feature have any kind of close relationship with each other.)
The borders of Mormonism are much less permeable; the practices of those who are not communicants, whose sacraments we don’t recognize and who don’t recognize our sacraments, will always be viewed with suspicion. The same innovations that make us so different also make us much less receptive for outside influences, with the result that we are conservative in some things where the rest of the Christian world has moved on. For example, like most churches in the 19th century but few in the 21st, the King James translation is our Bible of choice. Whether that changes depends nearly entirely on the needs of Mormons to communicate with each other, and very little on considerations of inter-religious dialogue. The delay in adopting other churches’ practices is only a problem for those who judge the Church by its adherence to the most recent trends in religion, or for those who imagine that it is never affected by cultural influences at all.