Those familiar with Church history and the teachings of the prophets are puzzled as to why so many LDS scholars teach that Cumorah is in Mexico. (This is the Mesoamerican/Two Cumorahs theory, or M2C, described below.) The short answer: M2C has become a tradition, supported by bias confirmation.
Bottom line: people can believe whatever they want. Basic psychology requires people to confirm their biases by selectively filtering the information they accept.
Those who seek to corroborate the teachings of the prophets about Cumorah have an abundance of evidence to support their views, including evidence from Church history, the text itself, and extrinsic scientific evidence.
Those who seek to reject the teachings of the prophets about Cumorah also have an abundance of evidence to support their views, including evidence from Church history, the text itself, and extrinsic scientific evidence.
Some who reject the teachings of the prophets about Cumorah still accept the Book of Mormon so they push M2C instead.
Others who reject the teachings of the prophets about Cumorah reject the Book of Mormon so push their theories that the Book of Mormon is fiction instead.
As always, belief is a choice. People who claim they are "following the evidence" are deluding themselves because in every instance, they choose to follow whatever evidence confirms their bias and reject whatever evidence contradicts their bias.
It shouldn't require much thought to realize that rejecting the teachings of the prophets about Cumorah undermines faith in the rest of the teachings of those same prophets. Long ago, Joseph Fielding Smith warned that M2C would cause members to become confused and disturbed in their faith. We see the evidence of that all around us.
Teaching M2C is not only puzzling, but when BYU and CES faculty teach M2C, it is a direct challenge to the Gospel Topics Entry which says, "the Church’s only position is that the events the Book of Mormon describes took place in the ancient Americas."
Of course, that position is not based on the text of the Book of Mormon, which never mentions America. It is based on the teachings of the prophets, and the same prophets who taught that the Book of Mormon took place in the ancient Americas also taught that Cumorah is in New York.
Nevertheless, these Church employees continue to teach M2C by telling students that the prophets, beginning with Joseph Smith and Oliver Cowdery, were wrong about Cumorah.
M2C is an integral part of their logo, which depicts a Mayan glyph to represent the Book of Mormon. BMC uses this logo to imprint M2C on the minds of everyone who reads their materials or watches their videos.
We love everyone at BMC. They have good intentions and most of what they do is awesome. They have accumulated lots of material to confirm their M2C bias. But M2C is unpersuasive to many faithful Latter-day Saints (not to mention to non-LDS Mayan experts, not to mention most of the world).
We continually encourage BMC to drop their dogmatic insistence on M2C and instead embrace the Church's position that accommodates multiple working hypotheses by all faithful Church members.
Past experience teaches that the BMC scholars will continue to promote M2C as the only acceptable setting for the Book of Mormon, but we can always hope for improvement.
Back to the original question: why do these faithful LDS scholars continue to promote M2C?
The simple answer is the scholars (beginning with RLDS scholars Henry A. Stebbins and memorialized in a map by L.E. Hills in 1917, later adopted by LDS scholars) assumed Joseph Smith taught that the Nephites lived in Central America, that the New York Cumorah doesn't meet their own subjective criteria for Cumorah, and that therefore Cumorah must be in southern Mexico. This leads them to conclude that the New York Cumorah was a false tradition that Joseph and his associates adopted, thereby misleading the Church and its members for over 150 years until the scholars figured out the truth.
Thus, they have "two Cumorahs," a false one in New York and the true one somewhere in southern Mexico.
Those who know Church history and read the text carefully can see the fallacies of the M2C assumptions, but that doesn't matter. M2C is now widespread because our LDS scholars have taught M2C for decades at BYU and in CES. M2C has become the filter through which BYU and CES students read the text.
The scholars effectively canonized the John Sorenson maps by publishing them widely and keeping them on the BYU Studies web page, where they still reside. For years they were on the splash page, but now at least they're buried under "Further Study," where you can see them here.
This one shows you exactly where (in Mexico) Cumorah is supposed to be. It is definitely not in New York.
These LDS scholars have persuaded themselves not only that the prophets were wrong, but that they, the scholars, should change and modify the text to incorporate such essential elements as volcanoes and Mayan temples.
The fantasy map corrects one of the major problems with M2C (the north/south orientation) but still teaches students that the prophets have been wrong about the New York Cumorah.
Worse, though, it teaches students to think of the Book of Mormon as taking place in a fantasy world, not even on the American continent.
Some historians overlook what was once common knowledge. Common knowledge becomes lost when people don't document it, or when historians ignore the documents. Some years ago I was teaching college students who had to look on their phones to figure out if the Vietnam war was before or after World War II. Both were history to them, something you read in books. To those citizens who lived during the Vietnam war, it was common knowledge that the Vietnam war was after WWII.
The New York Cumorah was so well known during Joseph Smith's lifetime that people didn't make a point of reporting about it, except in passing. Even in Letter VII, Oliver Cowdery declared it a fact but did not see the need to explain how he knew it was a fact because it was such common knowledge. Same with Joseph when he wrote what became D&C 128:20.We can see from the historical documents that Moroni identified the hill in New York as Cumorah the very first night he met Joseph Smith. Joseph's family, all of the Three Witnesses, and early members such as Parley P. Pratt, Heber C. Kimball, and Brigham Young all knew the hill where Joseph found the plates was the same Cumorah mentioned in Mormon 6:6. Church leaders who succeeded Joseph taught the New York Cumorah consistently and persistently through at least the 1970s, including members of the First Presidency speaking in General Conference.
M2C is the theory that claims
(i) Joseph adopted a false tradition that the hill in New York was the Cumorah of Mormon 6:6.
(ii) The "real" Cumorah of Mormon 6:6 is somewhere in Southern Mexico.
(iii) This means there are "Two Cumorahs," a false one in New York and a real one in Mexico.Remember, every time you see the logo of Book of Mormon Central you are seeing M2C.