There are dozens of different reasons given...there are hundreds of testimonies such as these below on the internet. For some it is study of the Bible itself, laying aside previous ideas, that leads to leaving the church. For some it is tied to history (Mountain Meadows Massacre, polygamy, view of blacks); for some it is contradictions between the revelations; for some it is the doctrine of God; for some it is the Book of Abraham fraud; for some the changes in the Book of Mormon and first vision story, and there are other reasons as well. But a common thread seems to be that something is wrong or missing spiritually; MANY say they could not measure up to the requirements. It is interesting to me how many former Mormons WANT to post their testimonies. I believe it is because they have found something better than Mormonism and want to share it.
For a very scholarly survey of why Mormons have left the church that is presented in a video by a man who is still in the church, visit Mormonstories.org, and watch "Top 5 Myths and Truths About Why Committed Mormons Leave the Church" The insights given there are well organized and some of them surprising!
Terry Foss Ex- Missionary
Out of Polygamy Into Christ
Evelyn Hall Lutheran...JW...and a cross
Amanda Computer Lab Witness
Suzanne Reading the New Testament
Robin Cohen Doubts after visiting the Temple
Timothy Sellman Ira's Security
Sherry Brown A Changed Husband
Derick Hartshorn Polygamy Question
John and Jenn Book of Mormon Doubts
Ben's story Book of Abraham triggers search
Kyle Sharpe Mormonism didn't change his life