Well, I think there are a couple of factors. Living within shouting distance of a non power 5 power house BSU I think I may have a little insight.
Boise St. has fairly low academic entrance standards which helps them recruit marginally academic athletes. They take a lot of tweeners, guys that aren't ready made to play especially linemen. For example their starting left tackle was a 6'5" 250 lb kid that was a low 3 star out of Texas. He now weights about 310. He is a good athlete that needed to get bigger and stronger and is now a 2 year starter and has a chance to play in the NFL.
BSU also recruits California very well, looking for guys that the elite PAC 12 schools don't want for what ever reason. Usually it is because they are a little undersized. Great athletes that make great corners or linebackers. BSU has a lot of 5'9 corners with chips on their shoulders and a lot of 6' 220 linebackers with chips on their shoulders. Some of these guys will get offers from Washington St. or Oregon St, but frankly BSU is a better offer.
BSU has good coaching it is almost like Wisconsin in that they don't change the formula no matter who comes into be the head coach. Brian Harsin is from the Peterson, Dan Hawkins, Dirk Keotter coaching tree.
Finally they don't play a meat grinder schedule. They play a couple of power 5 teams a year that are spaced out. They play one at the beginning of the year and then maybe one at the end. This year they actually played 2 at the beginning and they were Washington and Virginia. Not 2 big powers right now. There is no way they could survive playing Oregon, Washington, USC, Utah, Arizona 5 weeks in a row. They may go 3-2 at best through that. Too many good athletic teams. Playing through the BIG may be even worse because it is a more physical style. Think about playing Iowa, MSU, Nebraska, Minnesota, and OSU all in a row.
BSU is the big dog in there conference. I know they lost to Utah St on Friday giving up 8 turnovers, but USU is a pretty good team.