Brief
Chapter 2, where there's some interesting data
From Chapter 2:
The unaffiliated group provides a good example of the high degree of religious movement that has taken place in the U.S. Overall, 3.9% of the adult population reports being raised without any particular religious affiliation but later affiliating with a religious group. However, more than three times as many people (12.7% of the adult population overall) were raised in a particular faith but have since become unaffiliated with any religious group.
A similar dynamic is at work within Catholicism, but with very different results. Overall, 2.6% of
the U.S. adult population has switched their affiliation to Catholic after being raised in another faith or in no faith at all. But nearly four times as many people (10.1% of the adult population overall) were raised in the Catholic Church but have since left for another faith or for no faith at all.
I'd also suggest checking out the table on page 29.
Whatever the causes are, it seems it's something that is more complex than "Mormonism is a sick religion".
I think one aspect of this that is unique to Mormonism over traditional Christian faiths is the boundary between them. The boundary between various forms of Protestantism is much more permeable than between Mormonism and other faiths. I suspect a person who is socially unhappy with a congregation outside of Mormonism has more options than the Mormon who is unhappy in their ward. That by itself probably acts as a buffer to more extensive crisis of faith that might lead to a full-on paradigm shift.