“If the ‘keystone’ of Mormonism [The Book of Mormon] was delivered wrapped in fabrications, regarding it as nevertheless somehow ‘true’ becomes—to put it mildly—much more difficult.” 

Daniel Peterson, Literally hard facts at the beginning of Mormonism (Salt Lake City, UT: Deseret News, 03 December 2015).

For a brief glimpse into these reasons, please read all entries of “Defector of the Faith: 2,001+ Easy Reasons to Dismiss Mormonism.”