TLDR:
1. Strong foreign language skills from overseas missions.
2. It's easier getting them security clearances since they don't use drugs or alcohol.
replies(6):
Having grown up Mormon, I'd add a 4th that wasn't mentioned in the article: deference to authority. The Mormon church is a very hierarchal organization. Orders go from the top down. People lower on the ladder should not ever contradict their leaders. When a Mormon leader asks you to do something, you expected to comply. I imagine that this mindset would make you fit in well in the FBI, at least in the lower and middle layers of the organization.