I would speculate that most freshman don't play because of physical development (i.e. strength). Most players should be understanding the defensive concepts by this point in the season. The Big Ten is a physical league and the physical size/strength of the player plays a big role. There are athletes that can overcome their shortcomings and play but the majority need time to physically and mentally develop.
With all the top notch coaching in this league, any player's weakness (inexperience, strength, lack of discipline) is exploited quickly. For example, in today's game versus MSU, Izzo attacked our young front line (Oturu, Omersa) and got them to over commit defensively for easy dump-off layups. That is purely experience in replacing what you have always done defensively (i.e. natural tendencies) with what you have to do in a team defense (i.e. trust in your teammates). The development of a trusting defense is the key and that is a learned skill. Both Oturu and Omersa are continuing to get better but even as sophomores they are not there yet. Top ten teams have the depth, strength and experience to take advantage of lesser teams. The Gophers will get there with good recruiting but we are currently unfortunate with injuries (i.e. Curry) in providing us with critical depth/experience at a key position. Hurt is a good player but does not have the lateral quickness to provide significant backup minutes during league play.
I trust the coaches to know when a player is ready to compete at this level. Development is more than lifting or practice time. Building up a player to handle the mental aspects of the game has to be the focus and for most players that takes significant time. Playing someone too early can wreck their confidence and damage their college career. That is the "art" of coaching to know when to play them. Hopefully, the Gophers can continue to recruit the right players that will pan out on all those aspects. It would be nice to recruit five-star players that could play right away but very few teams get that luxury.