“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.”