That's because Bo understands (like Osborne) that most high school players really need a full year in the "Husker System" to understand the schemes and plays.
Remember LaVonte David's growth from his junior to senior years? He was a freak athlete as a junior and made a ton of big plays, but he was skating by on athleticism and not his brains or understanding of the system. As a senior he became an unstoppable force because he "got" the concepts.
Many teams trade that extra year of development away in favor of a freak freshman who has big athletic talent. But unfortunately, there's a down side, too. Freshmen who come in an unseat seniors can develop a touch of entitlement and may not give 100% of their mental devotion to the team. Kids who are forced to spend a year on the scout team develop a work ethic. And it weeds out the hard workers from the showboaters.
I'd gladly give up every spot on every freshman phenom list ever - if it means grabbing more spots on all-conference and all-American lists, and winning titles.