Question:

Why the Hill Cumorah must be in Mesoamerica, (southern Mexico and Central America), according to what the Book of Mormon says?

Answer:

The 43 soldiers of King Limhi traveled from the highlands of Guatemala, or Brazil if you prefer that scenario, looking for Zarahemla, which was only about 150 miles away. They ended up at the hill Cumorah where they located the remains of the Jaredite battle.  Since they thought they had actually located the land of Zarahemla it is rather hard to envision them walking all the way to upstate New York, a distance of 1,500 miles and amid the snowy weather, thinking they had been to Zarahemla.