of reasons.

1. At most schools, even elite ones, grads tend to work and get jobs in the area/region in which their school is located. Probably the case for BYU as well, but because BYU is located in a relatively small population center, with no substantial Fortune 500 presence, it sends (or attempts to send) its kids to bigger markets.

2. As I mentioned above, many BYU/LDS kids attend grad/professional schools in places like Chicago/Boston/DC/New York/LA. They then get hired to work in those places (see point #1 above), only to want to return west after a few years.

I live east of the Mississippi, and my ward has a LOT of LDS kids out here attending dental school. Most of these students (and their wives) project an attitude of, "it's fun to live here for a few years, but we would never even think of settling down here." For students, that's fine, because their schools don't really care where they go (in fact, the schools like it, as it expands their alumni network). But companies don't want to hire kids with attitudes like that.