Not sure why this was bumped from 2014 to make the comment that was made but since this is on page 1, here's my take:
1. Utah has some of the best male high school talent in the US. A Utahn won NXN 3 yrs in a row until this past fall.
2. BYU gets 90-95% of all top recruits from the state of Utah. The other in-state schools get the leftovers
3. BYU holds a roster of 30 men, the WORST of which was probably around a 4:15/9:10 guy in high school
4. They also have a club team of even MORE 4:15/9:10 guys who couldn't make the cut right away. This team is coached by one of their staff members and is as close to an actual NCAA team as they could get. Also, the boundary between being on the club/development team and the actual team is very fluid and kids get moved from one to the other if performance level changes.
5. BYU trains very hard. Mileage is on the high side. They have no problem smashing workouts. If you're on top in Provo it's because you deserve it.
6. BYU kids live a very clean lifestyle which helps a lot when it comes to staying healthy and improving
7. Team culture is very positive. Kids dream of going to BYU and are very passionate. This includes out-of-state LDS kids
8. Ed is a good coach. I wouldn't call him one of the best (hard to do so when you can't get your guys ready for NCAAs on a regular basis) but he is definitely a good coach. That being said, if you have 45 top-shelf guys battling for a coveted roster spot they pretty much coach themselves by racing every workout. You lose some talented kids but 7 makes a squad, right?
9. Altitude matters and while Provo isn't in that goldilocks zone over ~7000 ft it's still beneficial.
10. The missionary age thing is overrated. I've read studies that say that physical abilities (strength/power) tend to begin declining by then but the constant aerobic development will overcome that slight drop in capacity to reach a peak in the late 20's. If they aren't training on their missions (which they aren't) then it's a detriment. Not to mention there is a decent attrition rate due to loss of running-related priorities, etc.
11. What the mission DOES do is accelerate emotional development. If you drop an 18 yr old kid into a place like the Philippines and tell him to wear a shirt and tie in that climate while getting yelled at for two years - and PS you've got to learn Tagalog NOW - he's either going to crack or grow up real fast. These returned missionaries may be 24-25 when they graduate but they have the emotional age of a 30 year old. Compare that with your still-on-your-parents-cell-phone-plan 23 yr old from State U or wherever and you can see the advantage the mission gives.
Conclusion: if you have access to 45 kids with high school times under 4:15 in the mile who are extremely dedicated, live clean lives, and are emotionally very strong then train them at moderate altitude under a good coach at higher mileage on a team with very strong team culture (and perhaps a bit cut-throat) then you almost are guaranteed a top 5 team year-after-year. Not a mystery. Just statistical odds.